Type hinting for lru_cache on methods

[maxfischer2781]

This PR improves the type hinting of lru_cache. Major changes include:

• Type hint lru_cache as a descriptor to adjust parameters when used as a method
  • This erases all parameters for methods. This is similar to the typeshed hints for the standard library, but actually preservers parameters when not used as a method.
• Add type-tests in addition to unit-tests

Closes #104.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02 🎉

Comparison is base (ae606a9) 99.25% compared to head (749e428) 99.28%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #105      +/-   ##
==========================================
+ Coverage   99.25%   99.28%   +0.02%     
==========================================
  Files          20       20              
  Lines        2017     2225     +208     
  Branches      308      368      +60     
==========================================
+ Hits         2002     2209     +207     
  Misses         10       10              
- Partials        5        6       +1     

Impacted Files	Coverage Δ
unittests/test_functools.py	96.17% <100.00%> (+0.28%)	⬆️

... and 2 files with indirect coverage changes

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