New check: pinned Python dependency count

[jmbowman]

Add a check for the number of Python dependencies which are pinned in requirements/constraints.txt. The one for the edx-lint repository should also count the number of dependencies pinned in common_constraints.txt.

There may also be misplaced pins in setup.py, other requirements files, etc. I think we can safely ignore those for now, until we've quantified and addressed the known problem of excessive pins in the main constraints files.

[huniafatima-arbi]

PR adding the check for pinned python dependencies
@UsamaSadiq