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/mypy.ini

@@ -0,0 +1,19 @@
+[mypy]
+
+[mypy-deltachat.capi.*]
+ignore_missing_imports = True
+
+[mypy-pluggy.*]
+ignore_missing_imports = True
+
+[mypy-cffi.*]
+ignore_missing_imports = True
+
+[mypy-imapclient.*]
+ignore_missing_imports = True
+
+[mypy-pytest.*]
+ignore_missing_imports = True
+
+[mypy-_pytest.*]
+ignore_missing_imports = True