Pin importlib_metadata<5.0 for python <= 3.7 in requirements.txt (#2091)

v5.0.0 causes issues with python 3.7: python/importlib_metadata#411

Author: Balandat

requirements.txt

@@ -29,6 +29,10 @@ ax-platform
 nbformat>=4.2.0
 deep_phonemizer==0.0.17
 
+# the following is necessary due to https://github.com/python/importlib_metadata/issues/411
+importlib-metadata < 5.0; python_version <= "3.7"
+importlib-metadata; python_version > "3.7"
+
 # PyTorch Theme
 -e git+https://github.com/pytorch/pytorch_sphinx_theme.git#egg=pytorch_sphinx_theme