Move PymatgenTest.TEST_FILES_DIR attribute into module scope

[janosh]

e734bf3 fix IonEntry doc str + type hints
66d0043 move TEST_FILES_DIR attr from PymatgenTest into module scope
ca3c9e3 PymatgenTest.TEST_FILES_DIR -> TEST_FILES_DIR

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 74.57%. Comparing base (9455830) to head (fbe07e3).
Report is 1384 commits behind head on master.

Files with missing lines	Patch %	Lines
pymatgen/io/vasp/inputs.py	50.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3227   +/-   ##
=======================================
  Coverage   74.57%   74.57%           
=======================================
  Files         230      230           
  Lines       69493    69493           
  Branches    16166    16166           
=======================================
+ Hits        51823    51824    +1     
+ Misses      14596    14595    -1     
  Partials     3074     3074           

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