Update sync to return non-zero exit code on dry run

[francisbrito]

Resolves #1166

Description
Update sync function and CLI script so that they return non-zero exit codes if they're dry-run option is given.

Changelog-friendly one-liner: Update sync to return non-zero exit code on dry run

Contributor checklist

• Provided the tests for the changes.
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md on release).
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[francisbrito]

Notes & suggestions:

• In order to improve legibility and avoid making mistakes, I'd suggest that exit codes be abstracted into module-level variables. This way, tests and script code can be refactored to use said variables instead of relying on "magic numbers", e.g:

EXIT_CODE_SUCCESS = 0
EXIT_CODE_ERROR = 2

• The exit code to be returned when --dry-run is provided is 2. This is in line with what was requested in Feature: pip-sync --dry-run should return a non-zero exit code if changes were to occur #1166. If it makes more sense to return an alternative exit code for this case in particular. Please let me know so.

[codecov]

Codecov Report

Merging #1172 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1172   +/-   ##
=======================================
  Coverage   99.50%   99.51%           
=======================================
  Files          36       36           
  Lines        2852     2860    +8     
  Branches      335      335           
=======================================
+ Hits         2838     2846    +8     
  Misses          8        8           
  Partials        6        6           

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

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[atugushev]

I think code 1 would be more appropriate to not clash with code 2, which is used by click in the usage error context.

Yeah, pre-defined constants are good 👍

[atugushev]

Hey @francisbrito!

Thanks for your contribution! I took the liberty to address review comments and push changes to your branch since I'm going to release it soon. Feel free to fix "magic numbers" in a follow-up PR.

[atugushev]

LGTM 👍

[atugushev]

Thanks @francisbrito for your first contribution! Good job!

[francisbrito]

@atugushev You're welcome!

(PS: Sorry for taking so long to reply, had a busy past week)