rename master to main to make the history of the project easier to understand

CHANGES.rst

@@ -325,7 +325,7 @@ New features:
 - No longer test on Python 3.4, 3.5 and PyPy2, because we cannot get it to work.
   Technically it should still work, it is just no longer tested.
   Do not expect much development on branch 4.x anymore.
-  The master branch will be for the remaining Python versions that we support.
+  The main branch will be for the remaining Python versions that we support.
   [maurits]
 
 Bug fixes:
@@ -965,7 +965,7 @@ Fixes:
   [garbas]
 
 - hook out github repository to https://readthedocs.org service so sphinx
-  documentation is generated on each commit (for master). Documentation can be
+  documentation is generated on each commit (for main). Documentation can be
   visible on: https://icalendar.readthedocs.io/en/latest/
   [garbas]