fix: allow validations to work with dates/datetimes

[rich-iannone]

This makes it somewhat easier to put together column-to-column validations with dates/datetimes. Now, this works with several of the validation methods. A series of followup PRs will make it work with comparisons against a static date or datetime, but this particular PR solves #106.

Fixes: #106

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Project coverage is 99.03%. Comparing base (8ead09b) to head (f1e1373).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pointblank/validate.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #110      +/-   ##
==========================================
- Coverage   99.09%   99.03%   -0.06%     
==========================================
  Files          17       17              
  Lines        3524     3534      +10     
==========================================
+ Hits         3492     3500       +8     
- Misses         32       34       +2     

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