Python 3.10 support

[corranwebster]

Some issues have been reported with Python 3.10.

• "lvalue required as increment operand" error when building enable for Python 3.10 #949

[itziakos]

This is probably a old issue that has been fixed in previous version but with #1070 the ets workflow builds and tests enable on Python 3.10 so we should see issues like that anymore.