Rename nbgitpuller repo's master branch to main

[consideRatio]

Part of work in jupyterhub/team-compass#412 to rename our repo's master branches to main.

Actions to take, in order:

• approve this PR
• rename the master branch to main in GitHub (this should automatically update all PRs)
• merge this PR
• check the publish workflow publishes the expected outputs if relevant
• If readthedocs.org is used, set main as the default branch explicitly: admin -> advanced -> default branch

$ git grep master

CHANGELOG.md:- UI: Branch input placeholder no longer suggests master branch [#180](https://github.com/jupyterhub/nbgitpuller/pull/180) ([@sean-morris](https://github.com/sean-morris))
CHANGELOG.md:- Tell users about `main` vs `master` branches [#170](https://github.com/jupyterhub/nbgitpuller/pull/170) ([@yuvipanda](https://github.com/yuvipanda))

Comment: Changelog is fine!



binder/link_generator.ipynb:    "- **Default Values**: to avoid having to enter the same values in the widget's text fields on a repetitive basis, add the string values to the function's parameters. For example, the `branch` parameter defaults to `master`."
binder/link_generator.ipynb:      "text/plain": "'https://my.hub.com/hub/lti/launch?next=%2Fuser-redirect%2Fgit-pull?repo%3D%26branch%3Dmaster%26urlpath%3Dlab%252Ftree%252F.%252F%253Fautodecode'"
binder/link_generator.ipynb:    "def make_launch_link(is_assignment_link=True, is_jupyterlab=True, is_lti11=True, branch='master', hub_url='https://my.hub.com', repo_url='', urlpath=''):\n",

Comment: If the link generator references the master branch or not is irrelevant to the nbgitpuller project's use of a main or master branch I think.



docs/conf.py:github_doc_root = "https://github.com/rtfd/recommonmark/tree/master/doc/"

Comment: Just a link.



docs/conf.py:root_doc = master_doc = "index"

Comment: root_doc is now an option to master_doc supported by a modern version of sphinx, so setting both is a compatibility strategy.



docs/contributing.md:for it as well. A mix of [reStructuredText](https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html)

Comment: Just a link.



docs/link.rst:                 <input name="branch" id="branch" type="text" class="form-control" value="master" aria-label="Branch Name" aria-describedby="branch-prepend-label">
docs/link.rst:                    Use <code>main</code> instead of <code>master</code> for
docs/link.rst:                 <input name="content-branch" id="content-branch" type="text" class="form-control" value="master" aria-label="Branch Name" aria-describedby="content-branch-prepend-label">

Comment: these make sense.



docs/topic/repo-best-practices.md:gitignore](https://github.com/github/gitignore/blob/master/Python.gitignore)

Comment: Just a link.



docs/topic/url-options.rst:Branch in the git repo to pull from. Defaults to ``master``.

Comment: correct documentation.



nbgitpuller/pull.py:        # Merge master into local!
tests/test_api.py:            'branch': 'master',
tests/test_api.py:        assert '--branch master' in s
tests/test_api.py:            'branch': 'master',
tests/test_api.py:            'branch': 'master',
tests/test_gitpuller.py:        self.git('push', 'origin', 'master')
tests/test_gitpuller.py:    def __init__(self, remote, path='puller', branch="master", *args, **kwargs):
tests/test_gitpuller.py:            assert puller.git('name-rev', '--name-only', 'HEAD') == 'master'
tests/test_gitpuller.py:    branch = "master"
tests/test_gitpuller.py:            assert puller.gp.branch_exists("master")
tests/test_gitpuller.py:            assert puller.gp.resolve_default_branch() == "master"
tests/test_gitpuller.py:            pusher.git('push', 'origin', 'master')
tests/test_gitpuller.py:            pusher.git('push', 'origin', 'master')
tests/test_gitpuller.py:            pusher.git('push', '--force', 'origin', '%s:master' % orig_head)

Comment: nbgitpuller acts against master rather than main by default still, this PR only changes nbgitpuller's own code base location, so the entries above is okay.

[consideRatio]

@manics @choldgraf I went ahead and pinged you for a review hoping to get help making sure this doesn't get stale. It touches a few files and I hope to not have to rebase this and forget about the changes making a merge in the future harder and harder as time goes.

[yuvipanda]

@consideRatio thanks for working on this. I followed the checklist and I think everything is done. I'm not sure how the publish workflow might be affected by this rename, so I've left it unchecked. We use gh-pages for documentation so that seems fine...

[choldgraf]

Thanks all for working on this :-)