Renaming master branch to main

[dopplershift]

Now that GitHub supports renaming existing branches, we're planning on renaming the main development branch on GitHub from master to main. This follows a general trend in projects moving away from using master as the default branch because the term is considered hurtful to some people.

With GitHub's improved support, existing Pull Requests will not require any modifications. The only action required by users who use or contribute code on MetPy's development branch is that:

• Update local clones point to main rather than master so that e.g. git pull will continue to get the latest changes. GitHub's documentation indicates that git requests to the renamed branch should trigger a message.
• New Pull Requests should target main; it is likely that this should work by default and not require any manual intervention.

This rename is planned to take place some time on 16 February 2021.

[dopplershift]

master has been renamed to main as of 22 February 2021. All internal references to master were also updated in #1725.