* Split cgi.pyi into Py2 and 3 versions
* Restore correct Python 2 version of cgi.pyi
* Remove unnecessary version checks in cgi.pyi
* Use collections.abc
* Use List in Python 2 stub
A security fix added a "separator" argument to several URL parsing
functions and method in point releases:
* 3.6.13
* 3.7.10
* 3.8.8
* 3.9.2
Until all these versions are available on the GitHub Actions runners,
we need to whitelist the functions in the stubtests.