These are more "dirty" than the previous batch (that's why they were
left out). The trouble is that `compile` can return either a code object
or an AST depending on a flag, so we need to add an overload to make the
common case Union free. But it's still worthwhile.
Source was previously iterable because it implements `__getitem__()`,
which is apparently a thing from before `__iter__()` was introduced.
To reduce mypy's and my own confusion, implement `__iter__()` directly.
Add some Python 3.8 type: ignores; all are already fixed in the next
mypy release, so can be removed once we upgrade.
Also move some flake8 ignores which seem to have changed places.
Add a very lax mypy configuration, add it to tox -e linting, and
fix/ignore the few errors that come up. The idea is to get it running
before diving in too much.
This enables:
- Progressively adding type annotations and enabling more strict
options, which will improve the codebase (IMO).
- Annotating the public API in-line, and eventually exposing it to
library users who use type checkers (with a py.typed file).
Though, none of this is done yet.
Refs https://github.com/pytest-dev/pytest/issues/3342.