Use Protocol from typing_extensions for pre-3.8 Python

This commit adds an extra dependency to PySTAC when installed in
environments with versions of Python before 3.8. The 'Protocol' class
in `typing` was made available in the core module as of 3.8, but
previous versions can use it from the `typing_extensions`
package. setup.py is changed to install typing_extensions for pre-3.8
versions, and a conditional import is used to import the Protocol
class from the correct package.

Author: lossyrob

pystac/summaries.py

@@ -1,3 +1,4 @@
+import sys
 import json
 import numbers
 import urllib.request
@@ -17,10 +18,14 @@
     cast,
     TypeVar,
     Iterable,
-    Protocol,
     TYPE_CHECKING,
 )
 
+if sys.version_info >= (3, 8):
+    from typing import Protocol
+else:
+    from typing_extensions import Protocol
+
 if TYPE_CHECKING:
     from pystac.item import Item as Item_Type
     from pystac.collection import Collection as Collection_Type

setup.py

@@ -27,7 +27,10 @@
     packages=find_packages(),
     py_modules=[splitext(basename(path))[0] for path in glob('pystac/*.py')],
     include_package_data=False,
-    install_requires=["python-dateutil>=2.7.0"],
+    install_requires=[
+        "python-dateutil>=2.7.0",
+        'typing_extensions >= 3.7; python_version < "3.8"',
+    ],
     extras_require={
         "validation": ["jsonschema>=3.0"],
         "orjson": ["orjson>=3.5"]