Add support for PEXPIRE command's option

[dogukanteber]

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Description of change

closes #1938

[codecov-commenter]

Codecov Report

Merging #2026 (f04b16b) into master (6df0019) will decrease coverage by 0.06%.
The diff coverage is 45.16%.

@@            Coverage Diff             @@
##           master    #2026      +/-   ##
==========================================
- Coverage   92.17%   92.10%   -0.07%     
==========================================
  Files         100      100              
  Lines       21024    21053      +29     
==========================================
+ Hits        19379    19391      +12     
- Misses       1645     1662      +17     

Impacted Files	Coverage Δ
tests/test_commands.py	98.49% <34.78%> (-0.51%)	⬇️
redis/commands/core.py	85.79% <75.00%> (-0.07%)	⬇️

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[chayim]

@dogukanteber WDYT about making these options (NX, XX, etc) variables in the function header. IMHO it would watch our other functions.

[dvora-h]

@dogukanteber I just pushed a small fix (hope you don't mind) so the options will be consistent with options in other functions, now it's ready and we'll include it in the next release, thank you!