Add typing and enforce checking via tox/CI

This uses the same mypy settings as wsproto.

The Sentinel values are problematic, but I've found no good solution
that also has the property that type(Sentinel) is Sentinel - so this
should suffice for now.

Whilst I've been lazy with the tests, I've mostly avoided type ignores
in the main code. This should ensure that mypyc can be used if
desired.

h11/__init__.py

@@ -6,16 +6,57 @@
 # semantics to check that what you're asking to write to the wire is sensible,
 # but at least it gets you out of dealing with the wire itself.
 
-from ._connection import *
-from ._events import *
-from ._state import *
-from ._util import LocalProtocolError, ProtocolError, RemoteProtocolError
-from ._version import __version__
+from h11._connection import Connection, NEED_DATA, PAUSED
+from h11._events import (
+    ConnectionClosed,
+    Data,
+    EndOfMessage,
+    Event,
+    InformationalResponse,
+    Request,
+    Response,
+)
+from h11._state import (
+    CLIENT,
+    CLOSED,
+    DONE,
+    ERROR,
+    IDLE,
+    MIGHT_SWITCH_PROTOCOL,
+    MUST_CLOSE,
+    SEND_BODY,
+    SEND_RESPONSE,
+    SERVER,
+    SWITCHED_PROTOCOL,
+)
+from h11._util import LocalProtocolError, ProtocolError, RemoteProtocolError
+from h11._version import __version__
 
 PRODUCT_ID = "python-h11/" + __version__
 
 
-__all__ = ["ProtocolError", "LocalProtocolError", "RemoteProtocolError"]
-__all__ += _events.__all__
-__all__ += _connection.__all__
-__all__ += _state.__all__
+__all__ = (
+    "Connection",
+    "NEED_DATA",
+    "PAUSED",
+    "ConnectionClosed",
+    "Data",
+    "EndOfMessage",
+    "Event",
+    "InformationalResponse",
+    "Request",
+    "Response",
+    "CLIENT",
+    "CLOSED",
+    "DONE",
+    "ERROR",
+    "IDLE",
+    "MUST_CLOSE",
+    "SEND_BODY",
+    "SEND_RESPONSE",
+    "SERVER",
+    "SWITCHED_PROTOCOL",
+    "ProtocolError",
+    "LocalProtocolError",
+    "RemoteProtocolError",
+)