CONTRIBUTING

typing: mark the argument to mercurial.i18n.gettext() non-Optional

Few if any of the callers are handling a `None` return, which is making pytype
complain.  I tried adding @overload definitions to indicate the bytes -> bytes
and None -> None relationship, but pytype doesn't seem to apply that to `_()`
through the function assignment.  What did work was to change `_()` into its own
function that called `gettext()`, but that has an extra function call overhead.
Even putting that function into an `if pycompat.TYPE_CHECKING` block and leaving
the existing assignments in the `else` block caused pytype to lose track of the
@overloads.

At that point, I simply gave up.  PyCharm doesn't like that it can return None,
given the new type hints, but pytype doesn't complain about this nor does it
see any callers passing None.  The most important thing here is to catch str
being passed anyway.

Differential Revision: https://phab.mercurial-scm.org/D10235

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