support new Python "soft keywords": match, case and _

[taleinat]

Python 3.10 introduces pattern-matching blocks, in which the match, case and _ "soft keywords" are considered keywords only in specific parts of the syntax. See the docs for more details.

However, the list of Python keywords in lexers/python.py doesn't include these, nor does the Python lexer appear to have any other way of supporting these keywords.

[nedbat]

@taleinat doesn't need to be told, but others might: here is how IDLE chose to support the soft keywords: python/cpython#25851

For coverage.py's HTML report, I used the ast module to find the start of match/case statements to help.