Ruggedize cmd2's runtime type annotation validation

[tleonhardt]

Change cmd2's runtime type annotation validation for hook callback function registration to be based on typing.get_type_hints

Previously it was based on inspect.signature. The problem is that prior to Python 3.10, the inspect module doesn't have a safe way of evaluating type annotations that works equivalently both in the presence or absence of from __future__ import annotations. Hence, any attempt at using that in an app would break cmd2. Even in Python 3.10+, it requires an extra argument to be provided.

This change adds a get_types() helper function to the cmd2.utils module which uses typing.get_type_hints() to do the introspection in a safer way.

Down the road when cmd2 drops support for Python 3.9, we can evaluate to switching the underlying implementation to use inspect.get_annotations() if we wish.

Closes #1443

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.55%. Comparing base (8d1c82c) to head (896c7bb).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1442      +/-   ##
==========================================
- Coverage   98.56%   98.55%   -0.02%     
==========================================
  Files          20       20              
  Lines        5726     5742      +16     
==========================================
+ Hits         5644     5659      +15     
- Misses         82       83       +1     

Flag	Coverage Δ
unittests	98.55% <100.00%> (-0.02%)	⬇️

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