Add typing to several areas where straightforward

[jdufresne]

Refs: #972

[codecov]

Codecov Report

Merging #1315 (6e8017f) into master (d9f50f1) will decrease coverage by 0.06%.
The diff coverage is 97.64%.

@@            Coverage Diff             @@
##           master    #1315      +/-   ##
==========================================
- Coverage   99.65%   99.58%   -0.07%     
==========================================
  Files          33       33              
  Lines        2889     2907      +18     
  Branches      320      321       +1     
==========================================
+ Hits         2879     2895      +16     
- Misses          5        6       +1     
- Partials        5        6       +1     

Impacted Files	Coverage Δ
piptools/resolver.py	98.12% <71.42%> (-1.24%)	⬇️
piptools/__main__.py	100.00% <100.00%> (ø)
piptools/cache.py	100.00% <100.00%> (ø)
piptools/exceptions.py	100.00% <100.00%> (ø)
piptools/logging.py	100.00% <100.00%> (ø)
piptools/repositories/base.py	100.00% <100.00%> (ø)
piptools/sync.py	100.00% <100.00%> (ø)
piptools/utils.py	100.00% <100.00%> (ø)
tests/test_cache.py	100.00% <100.00%> (ø)
tests/test_utils.py	100.00% <100.00%> (ø)

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[atugushev]

Only a minor comment below. Otherwise, LGTM 👍🏼