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/const.py

@@ -1,7 +1,13 @@
+from typing import Any, List
+
 from .capi import lib
 
 
-for name in dir(lib):
+def __getattr__(name: str) -> Any:
     if name.startswith("DC_"):
-        globals()[name] = getattr(lib, name)
-del name
+        return getattr(lib, name)
+    return globals()[name]
+
+
+def __dir__() -> List[str]:
+    return sorted(name for name in dir(lib) if name.startswith("DC_"))