Remove master branch references after migrating to main

[kenibrewer]

Fixes #318

Description

As part of the transition to more inclusive language this removes references to the master branches that were previously present in the codebase.

What is the nature of your change?

• Bug fix (fixes an issue).

Checklist

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• I have read the CONTRIBUTING.md guidelines.
• My code follows the style guidelines of this project.
• I have performed a self-review of my own code.
• I have deleted all non-relevant text in this pull request template.

[codecov-commenter]

Codecov Report

Merging #324 (fc6bbba) into main (74e6c63) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #324   +/-   ##
=======================================
  Coverage   95.40%   95.40%           
=======================================
  Files          56       56           
  Lines        3134     3134           
=======================================
  Hits         2990     2990           
  Misses        144      144           

Flag	Coverage Δ
unittests	95.40% <ø> (ø)

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[kenibrewer]

@d33bs Could I request a review on this please? Thanks!

[d33bs]

This LGTM, thank you @kenibrewer !

As an aside, I think it might be good to double check the config under https://readthedocs.org/projects/pycytominer/ to make sure it references main moving forward. @gwaybio , do you know if we're able to change what branch or reference URL is used for documentation there?

[gwaybio]

do you know if we're able to change what branch or reference URL is used for documentation there?

Done! Changed to main in RTD. Docs are passing!

[kenibrewer]

This is ready to merge.

[gwaybio]

Thanks @kenibrewer !