watchman: refactor `ctypes.windll.kernel32` references to a local variable This is flagged by pytype as an attribute-error, and it's easier to disable that in a single place.

typing: disable [unsupported-operands] warning in the largefiles outgoing hook For some reason, pytype thinks `toupload` is a set: No attribute '__setitem__' on Set[nothing] (It actually is a set in the subsequent `else` branch, but I'm not interested in trying to rewrite this to be consistent.)

typing: add some assertions that a variable isn't None In the case of blackbox, there's a default limit if one isn't explicitly supplied. For the monotone regex, neither group is optional, so a match means it's not None.

largefiles: reference `mercurial.configitems.dynamicdefault` directly Pytype was unable to see `dynamicdefault` on `eh.configitem`. This is clearer anyway.

releasenotes: fix a typo in a comment

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.

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

hgweb: skip body creation of HEAD for most requests The body is thrown away anyway, so this just wastes a lot of CPU time. In the case of /archive/, this skips manifest processing and the actual file archiving, resulting in a huge difference. The most tricky part here is skipping the Content-Length creation as it would indicate the output size for the corresponding GET request.