hghave: make black version regex work with newer versions of black

Black commit 117891878e5be4d6b771ae5de299e51b679cea27 (included in black >=
21.11b0) dropped the string "version " from the output of "black --version". To
make the regex work with newer black versions, make matching of "version "
optional.
16 jobs for topic/stable/black_version_regex in 89 minutes and 50 seconds (queued for 6 seconds)