Replace master with main where possible

[michael-k]

I've noticed that the default branch has been renamed from master to main. This PR

• cleans up all places that reference/refer to pip-tool's default branch,
• uses the branch main of other repos where possible (eg. actions/checkout), and
• let the tests use main instead of master for mocked repositories.

This does fix the linkcheck-docs GitHub Actions job.

The codecov badge doesn't work yet: I don't know what's necessary to fix that.

The word “master” remains in 3 places:

• pip-tools/.github/workflows/release.yml

  Line 39 in 6839a64

  uses: pypa/gh-action-pypi-publish@master

• pip-tools/docs/conf.py

  Line 1 in 6839a64

  # https://www.sphinx-doc.org/en/master/usage/configuration.html

• pip-tools/docs/index.rst

  Line 1 in 6839a64

  .. pip-tools documentation master file, created by

Contributor checklist

• Provided the tests for the changes.
• Assure PR title is short, clear, and good to be included in the user-oriented changelog

Maintainer checklist

• Assure one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[atugushev]

We should update this to release/v1:

pip-tools/.github/workflows/release.yml

Line 39 in 6839a64

uses: pypa/gh-action-pypi-publish@master

See https://github.com/pypa/gh-action-pypi-publish#-master-branch-sunset-

[ssbarnea]

@atugushev Yeah, I am fully aware of that isolated case...