Unify datetime imports

[akx]

Various modules in Babel imported datetime in various ways, sometimes needing aliases and whatnot; this commit unifies everything to import datetime.

[codecov]

Codecov Report

Merging #945 (fd97d5a) into master (14216ed) will decrease coverage by 0.01%.
The diff coverage is 93.93%.

@@            Coverage Diff             @@
##           master     #945      +/-   ##
==========================================
- Coverage   90.53%   90.52%   -0.01%     
==========================================
  Files          24       24              
  Lines        4395     4392       -3     
==========================================
- Hits         3979     3976       -3     
  Misses        416      416              

Impacted Files	Coverage Δ
babel/localtime/__init__.py	62.16% <90.00%> (ø)
babel/dates.py	85.02% <90.90%> (-0.07%)	⬇️
babel/localtime/_unix.py	30.76% <100.00%> (ø)
babel/localtime/_win32.py	62.96% <100.00%> (ø)
babel/messages/catalog.py	95.52% <100.00%> (ø)
babel/messages/frontend.py	87.75% <100.00%> (ø)
babel/numbers.py	98.00% <100.00%> (ø)
babel/support.py	85.09% <100.00%> (ø)
babel/util.py	97.32% <100.00%> (ø)

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[akx]

@DenverCoder1, could you take a reviewy look? :)

[DenverCoder1]

Looks good to me