Add support for b and B period symbols in time format

[jun66j5]

Fix for #866

[codecov]

Codecov Report

Merging #869 (38a0e42) into master (3ae5402) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #869      +/-   ##
==========================================
- Coverage   91.34%   91.29%   -0.05%     
==========================================
  Files          23       23              
  Lines        4192     4183       -9     
==========================================
- Hits         3829     3819      -10     
- Misses        363      364       +1     

Impacted Files	Coverage Δ
babel/dates.py	92.50% <100.00%> (-0.05%)	⬇️
babel/localtime/_unix.py	23.61% <0.00%> (-1.05%)	⬇️
babel/localtime/__init__.py	64.10% <0.00%> (-0.90%)	⬇️
babel/messages/checkers.py	80.00% <0.00%> (-0.25%)	⬇️
babel/messages/jslexer.py	86.66% <0.00%> (-0.15%)	⬇️
babel/messages/mofile.py	89.88% <0.00%> (-0.12%)	⬇️
babel/support.py	84.55% <0.00%> (-0.07%)	⬇️
babel/messages/frontend.py	87.91% <0.00%> (-0.05%)	⬇️
babel/messages/extract.py	94.87% <0.00%> (-0.04%)	⬇️
... and 13 more

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[akx]

Thank you, looks pretty good! Some minor comments within :)

[akx]

Thank you for the contribution and the fast review turnaround, looks great! :)