schemes: fix a broken check for drive letter conflicts
Flagged by pytype locally. It appears to have regressed in
1863584f2fba (not
yet released).
This seems like an obvious typo- `dict.isalpha()` is nonsense. There's no crash
though because `schemes` is pre-populated with 5 schemes (that are all now
defunct), so the length of the dict is never 1, so it's impossible to abort.
typing: suppress a bunch of potential import-error cases in extensions
As flagged by pytype locally. Either the ImportError is locally handled, or the
imported module was previously determined to be present by `hgave` (for the
phabricator extension), or is handled by the `hgext.convert.subversion` module
when imported (for the `hgext.convert.transport` module).