vcs: fix missing import of mercurial.localrepo (localrepo might already always be available in the mercurial namespace due to side effects from other imports, but it is still better to do it explicit ... and also to please pytype.)

pytest >= 4.6.6, < 4.7
pytest-sugar >= 0.9.2, < 0.10
pytest-benchmark >= 3.2.2, < 3.3
pytest-localserver >= 0.5.0, < 0.6
mock >= 3.0.0, < 3.1
Sphinx >= 1.8.0, < 1.9
WebTest >= 2.0.6, < 2.1
isort == 4.3.21