Add type hints for the writer module

[atugushev]

Refs: #972

[codecov]

Codecov Report

Merging #1310 (8f2755b) into master (4776ac9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1310   +/-   ##
=======================================
  Coverage   99.65%   99.65%           
=======================================
  Files          33       33           
  Lines        2880     2889    +9     
  Branches      320      320           
=======================================
+ Hits         2870     2879    +9     
  Misses          5        5           
  Partials        5        5           

Impacted Files	Coverage Δ
piptools/logging.py	100.00% <100.00%> (ø)
piptools/utils.py	100.00% <100.00%> (ø)
piptools/writer.py	100.00% <100.00%> (ø)

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[jdufresne]

Thanks for getting the ball rolling!

[atugushev]

Thank you for reviewing this @jdufresne 🙏🏼