Say you're attrs and you don't configure target-version. Previously
Black would default to trying every single grammar it has, including the
3.10 grammar. With this PR, now target-version is configured for you
effectively so now get_grammars() is more selective in which grammars it
returns. If any target versions don't support the match statement, the
3.10 grammar won't be tried.

And while in theory a 3.7+ project shoudn't be using 3.10 features,
attrs has a test file which uses match (which fails to parse because the
3.10 grammar isn't selected). To avoid breaking attrs, get_grammars()
will now return the 3.10 grammar as long as *any* the target versions
support match.

The out() call was made redundant by an older PR that prints the
configuration if --verbose is passed.