`cutil.from_dc_charpointer()` is guaranteed to return `str`, while
`cutil.from_optional_dc_charpointer()` may return `None` if C function
returns `NULL`.
`deltachat.const` module now defines `__getattr__` and `__dir__` as
suggested by https://www.python.org/dev/peps/pep-0562/
mypy detects that `__getattr__` is defined and does not show errors
for `DC_*` constants which cannot be detected statically.
mypy is added to `tox.ini`, so type check can be run with `tox -e mypy`.
A lot of work from @Hocuri and @Simon-Laux mostly.
This exposes the API of the deltachat-provider-overview crate on the
deltachat FFI API, allowing clients to use it to help users set up
their accounts.
