[3.6] Change six instances of "an" to "a"

[kms70847]

"An ModuleNotFoundError" appears several times in this document, likely because in the previous version it used to be "An ImportError".
"An path" also appears.
These instances of "an" should be "a" instead.

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[kms70847]

Initially I wasn't sure whether "an __path__" ought to be changed to "a __path__", since one might conceivably pronounce it as "an underscore underscore path underscore underscore", but a quick grep through my slightly outdated local copy indicates that the underscores are conventionally not pronounced:

c:\Programming\Python 3.5\source\Python-3.5.1\Doc>grep -rsi "a ``__" *
c-api/object.rst:   A generic implementation for the getter of a ``__dict__`` descriptor. It
c-api/object.rst:   A generic implementation for the setter of a ``__dict__`` descriptor. This
faq/programming.rst:the compiled code should be created in a ``__pycache__`` subdirectory of the
faq/programming.rst:has the ability (permissions, free space, etc...) to create a ``__pycache__``
faq/programming.rst:This will write the ``.pyc`` to a ``__pycache__`` subdirectory in the same
howto/pyporting.rst:an object defines a ``__truediv__`` method but not ``__floordiv__`` then your
library/codeop.rst:   text containing a ``__future__`` statement, the instance 'remembers' and
library/doctest.rst:* Define a ``__test__`` dictionary mapping from regression test topics to
library/functools.rst:   automatically adds a ``__wrapped__`` attribute to the wrapper that refers to
library/functools.rst:      function, even if that function defined a ``__wrapped__`` attribute.
library/idle.rst:an Editor window which are not via ``__main__`` and :data:`sys.modules` will
library/inspect.rst:   chain has a ``__signature__`` attribute defined.
library/plistlib.rst:      Dict values in the result have a ``__getattr__`` method that defers
library/plistlib.rst:      Dict values in the result have a ``__getattr__`` method that defers
library/runpy.rst:      Added ability to execute packages by looking for a ``__main__`` submodule.
library/runpy.rst:   compiled bytecode file or a valid sys.path entry containing a ``__main__``
library/zipapp.rst:   Write a ``__main__.py`` file to the archive that executes *mainfn*.  The
library/zipapp.rst:   contain a ``__main__.py`` file, as otherwise the resulting archive
library/zipapp.rst:Python has been able to execute zip files which contain a ``__main__.py`` file
library/zipapp.rst:simply has to be a standard zip file containing a ``__main__.py`` file which
reference/datamodel.rst:is prepared. If the metaclass has a ``__prepare__`` attribute, it is called
reference/import.rst:module.  Specifically, any module that contains a ``__path__`` attribute is
reference/import.rst:   Non-package modules should not have a ``__path__`` attribute.
reference/import.rst: * If the module has a ``__spec__`` attribute, the information in the spec
reference/import.rst: * If the module has a ``__file__`` attribute, this is used as part of the
reference/import.rst: * If the module has no ``__file__`` but does have a ``__loader__`` that is not
using/cmdline.rst:      Supply the package name to run a ``__main__`` submodule.
using/cmdline.rst:   containing a ``__main__.py`` file, or a zipfile containing a
whatsnew/2.6.rst:Python 2.6 has a ``__future__`` import that removes ``print`` as language
whatsnew/2.6.rst:There's also a ``__future__`` import that causes all string literals
whatsnew/3.1.rst:  a ``__main__`` submodule when a package name is supplied.
whatsnew/3.3.rst:Another change is that all modules have a ``__loader__`` attribute, storing the
whatsnew/3.4.rst:* Frozen modules no longer define a ``__file__`` attribute. It's semantically
whatsnew/3.5.rst:Without a ``__future__`` import, a :exc:`PendingDeprecationWarning` will be
whatsnew/3.5.rst:the files, including a ``__main__.py`` file, into a directory ``myapp``

c:\Programming\Python 3.5\source\Python-3.5.1\Doc>grep -rsi "an ``__" *
library/contextlib.rst:It is occasionally desirable to catch exceptions from an ``__enter__``
library/contextlib.rst:Cleaning up in an ``__enter__`` implementation
reference/import.rst:By definition, if a module has an ``__path__`` attribute, it is a package,
whatsnew/2.1.rst:  is used, by defining an ``__all__`` attribute containing a list of names that
whatsnew/3.4.rst:* The :class:`int` constructor now accepts any object that has an ``__index__``

Besides path, all of the other "an"s here precede a name starting with a vowel.

[brettcannon]

@kms70847 is this purposefully only against 3.6 and not master? Is master fixed for this?

[Mariatta]

The changes here should be applied to master too.
@kms70847 please make your PR against master branch. Once it's merged, it can be backported to 3.6.
Thanks.

[vadmium]

This looks okay to me.

The ModuleNotFoundError change was introduced with commit 46f97b8. For bonus points, the problem with an could also be fixed in a comment in “test.test_importlib.import_.test_fromlist.HandlingFromlist.test_module_from_package_triggers_ModuleNotFoundError”.

[brettcannon]

I tried changing the base to master through the web UI, but then it suddenly picked up all of the changes between 3.6 and master so that didn't work.

[Mariatta]

Please make the change against master branch.

[kms70847]

I have created a new pull request with identical changes except it's on the master branch: #1862

I could not quite figure out how to change the branch without creating a new PR. I apologize if this is an inconvenience.