API: Change Period('NAT') to return NaT

[sinhrks]

• closes API: Change Period('NAT') to return NaT, not Period instance #12759, Checking for NaT in PeriodIndex doesn't work with None, pd.NaT #13582
• tests added / passed
• passes git diff upstream/master | flake8 --diff
• whatsnew entry
• pd.Period('NaT', freq='D') now returns pd.NaT rather than its own nat repr.
• To be compat with Period arithmetic, pd.NaT now supports addition / subtraction with int (previously ValueError).

pd.NaT + 1
# pd.NaT

Added tests for following:

• Period and PeriodIndex, Series ops
  • add, sub, comp
• PeriodIndex creation from list-like which contains Period and NaT (ENH: PeriodIndex now accepts pd.NaT #13430)
• PeriodIndex boxing
• .to_period
• __contains__ any NaT-like(pd.NaT, None, float('nan'), np.nan)

[sinhrks]

CC @MaximilianR

[codecov-io]

Current coverage is 84.38%

Merging #13609 into master will decrease coverage by <.01%

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- Hits          43225      43224     -1   
- Misses         8000       8001     +1   
  Partials          0          0          

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[jorisvandenbossche]

Periond -> Period

[jorisvandenbossche]

@sinhrks Can you rebase this? Then it will be easier to review the additional changes (as I think the first commit is now already in master?)

[sinhrks]

@jorisvandenbossche sure. rebased.

[jreback]

thanks @sinhrks this is really nice!