djblets.siteconfig, and SiteConfiguration in particular, is central

to much of the Djblets and Review Board codebases. This change

inrtoduces type annotations to most of the module, helping to make sure

call sites are passing in all the right information.
Settings management in SiteConfiguration is all typed now, helping

callers ensure they're handling results correctly.
Mappings between siteconfig and Django settings are typed with

structured TypeDicts, so we can avoid any issues with invalid keys or

values in those sometimes-complex mappings.
This was fairly smooth, though it did uncover a couple code paths where

assumptions were incorrectly made. Unbound varibable issues were fixed

in the management commands

All unit tests pass for all supported versions of Python.
Mypy and pyright were largely happy. There's some work in related modules

that still need to be done.