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).

watchman: drop some py2 compat code The `unicode` reference was being flagged by pytype, even though it was never evaluated on py3. There's more that can be dropped and `compat.py` can probably be inlined if we don't care about minimizing the code changes from FB. But I don't feel like dealing with that.