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.

pytype: add coverage for hgdemandimport This would have flagged what needed fixing in 48e38b179106 long ago.