`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`.
- Use latest stable for rustfmt and clippy.
- Update Rust in rust-toolchain and coredeps Docker container to 1.54.0
and test against it.
- Test against 1.48.0 to ensure libdeltachat can be compiled with Debian
bullseye "rustc" and "cargo".
We don't really have docs in the other crates yet, and there's a
naming conflict between deltachat and deltachat_ffi that needs to
sorted to build both. But deltachat_ffi is still documented with
doxygen for now so there's no need to sort this now.
