Replace isort by native functionality to identify Python's stdlib modules

[Nour-Mws]

This issue is created to address a discussion in the comments of PR #8.
We're currently using the place_module function from the isort library to assess whether an imported dependency is part of Python's stdlib.
It turns out that isort has a static list of stdlib modules for each Python version. And on further digging in the isort code, @jherland found out that the way these lists are compiled is based on non documented features:

I followed your above link, and looked at the files themselves, which led me to the script that isort uses to make those files, and it's is even more horrible: There seems to be an objects.inv file available from python.org for each release (e.g. https://docs.python.org/3.7/objects.inv), but it's in some weird undocumented partially zlib-compressed format that is decoded by some utility inside Sphinx (a tool for generating API documentation). Yuck...
(edit: more information here: https://sphobjinv.readthedocs.io/en/stable/syntax.html)

Proposed solution

To replace the above isort function with native code, we could use sys.stdlib_module_names in Python >=v3.10, and keep hardcoded lists of stdlib modules for versions 3.7, 3.8 and 3.9.

[jherland]

Reclassified this issue as considerably less important (consider even wontfix): Since we created this issue we have come to rely more, not less, on isort's classification of import statements. If we are to drop isort as a dependency, it must come with additional benefits/rationale.