Renamed main branch from `master` into `main`

[forman]

Checklist:

• [ ] ~Add unit tests and/or doctests in docstrings~
• [ ] ~Add docstrings and API docs for any new/modified user-facing classes and functions~
• [ ] ~New/modified features documented in docs/source/*~
• [x] Changes documented in CHANGES.md
• [x] GitHub CI passes
• [x] AppVeyor CI passes
• [x] Test coverage remains or increases (target 100%)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.21%. Comparing base (aca84e1) to head (260836a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #949 +/- ## ======================================= Coverage 93.21% 93.21% ======================================= Files 458 458 Lines 35812 35812 ======================================= Hits 33382 33382 Misses 2430 2430 ```

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