feat(python)!: Remove class variables from some DataTypes

[stinodego]

Some DataType classes had class variables. The Datetime class, for example, had time_unit and time_zone as class variables. This was unintended: these should have been instance variables. This has now been corrected.

Example

Before:

>>> dtype = pl.Datetime
>>> dtype.time_unit is None
True

After:

>>> dtype.time_unit is None
Traceback (most recent call last):
...
AttributeError: type object 'Datetime' has no attribute 'time_unit'

Use instead:

>>> getattr(dtype, "time_unit", None) is None
True

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.50%. Comparing base (d7b4f72) to head (35fbace).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16524      +/-   ##
==========================================
- Coverage   81.51%   81.50%   -0.02%     
==========================================
  Files        1414     1414              
  Lines      185906   185899       -7     
  Branches     3027     3024       -3     
==========================================
- Hits       151535   151509      -26     
- Misses      33840    33861      +21     
+ Partials      531      529       -2     

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