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/direct_imap.py

@@ -11,7 +11,7 @@ from imapclient import IMAPClient
 from imapclient.exceptions import IMAPClientError
 import imaplib
 import deltachat
-from deltachat import const
+from deltachat import const, Account
 
 
 SEEN = b'\\Seen'
@@ -62,7 +62,7 @@ def dc_account_after_shutdown(account):
 
 
 class DirectImap:
-    def __init__(self, account):
+    def __init__(self, account: Account) -> None:
         self.account = account
         self.logid = account.get_config("displayname") or id(account)
         self._idling = False