feat: Implement LocalDateTime

[chrisimcevoy]

As usual, large portions of this type were already implemented as a by-product of previous ports of other types.

This adds all the missing bits, including the TestLocalDateTime.cs port (at least, as much as it makes sense to do so).

It also fixes a couple of lurking bugs in _TimePeriodField which were unearthed along the way.

[codecov]

Codecov Report

Attention: Patch coverage is 99.77974% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.59%. Comparing base (6224e4d) to head (e847026).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pyoda_time/_local_date_time.py	99.13%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #260      +/-   ##
==========================================
+ Coverage   96.46%   96.59%   +0.12%     
==========================================
  Files         263      263              
  Lines       20902    21318     +416     
==========================================
+ Hits        20164    20593     +429     
+ Misses        738      725      -13     

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