Add a comment in `setup.py` to clarify that `importlib-metadata` is only needed for python 3.7

[parthea]

We should add a comment to setup.py to clarify that importlib-metadata is only needed for compatibility with python 3.7

Also, the package name in PyPI is importlib-metadata but in setup.py we have importlib_metadata

For python 3.7, we use importlib-metadata from PyPI
For python 3.8, we use importlib.metadata which is builtin.

python-bigquery-storage/setup.py

Line 47 in fb0d6f0

"pandas": ["pandas>=0.21.1", "importlib_metadata>=1.0.0; python_version<'3.8'"],

[xionams]

Hi @parthea could you assign it to me? Thanks

[chalmerlowe]

Closing as finished. See linked fix PR above.