Add type hints for the sync module

[atugushev]

Refs: #972

[codecov]

Codecov Report

Merging #1331 (0475a76) into master (4e78766) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1331   +/-   ##
=======================================
  Coverage   99.62%   99.62%           
=======================================
  Files          33       33           
  Lines        2950     2953    +3     
  Branches      322      322           
=======================================
+ Hits         2939     2942    +3     
  Misses          5        5           
  Partials        6        6           

Impacted Files	Coverage Δ
piptools/scripts/sync.py	100.00% <100.00%> (ø)
piptools/sync.py	100.00% <100.00%> (ø)

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[atugushev]

Thanks @jdufresne 🙏🏻