python: add mypy support and some type hints

`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`.

python/src/deltachat/provider.py

@@ -14,7 +14,7 @@ class Provider(object):
     :param domain: The email to get the provider info for.
     """
 
-    def __init__(self, account, addr):
+    def __init__(self, account, addr) -> None:
         provider = ffi.gc(
             lib.dc_provider_new_from_email(account._dc_context, as_dc_charpointer(addr)),
             lib.dc_provider_unref,