Also unconditionally use the `mypy` from our virtualenv --
that's how we ensure we use a common version across different
Zulip developers and in CI.
And as a side effect of cutting some Python 2 vs. Python 3 logic,
fix a bug where `--all` was having no effect.
Notably, this adds our checks on translated message strings to
`tools/test-all`, so that they don't cause surprise failures in
CI after a branch is pushed. (Alternately they could have gone
in `tools/lint` to accomplish the same goal, but `makemessages`
which they depend on is quite slow -- on my machine it takes 7s,
compared to 10s for all of `tools/lint`.)
Previously, we checked scripts in a separate run to work around mypy
not supporting multiple scripts with the same name. Since we have
fixed that issue, we can restore the original behavior.
We leave the --scripts-only option available, though I'm not sure it's
particularly useful and we'll probably eventually remove it.
We're now at the point where 100% of functions checked by mypy is
fully annotated; to avoid regressions, we're enforcing the requirement
that it stay this way. We still have a moderate amount of code that
is neither checked by mypy nor annotated, but it seems reasonable to
annotate that code at the same time as we get a chance to fix the mypy
issues in it.
This is implemented by using the --disallow-untyped-defs option in
mypy by default.
