Add some tiny type annotations (#7870)

I found these made pycharm have more of a clue as to what was going on in other places.

changelog.d/7870.misc

@@ -0,0 +1 @@
+Add some type annotations to `HomeServer` and `BaseHandler`.

synapse/handlers/_base.py

@@ -17,6 +17,8 @@
 
 from twisted.internet import defer
 
+import synapse.state
+import synapse.storage
 import synapse.types
 from synapse.api.constants import EventTypes, Membership
 from synapse.api.ratelimiting import Ratelimiter
@@ -28,21 +30,17 @@
 class BaseHandler(object):
     """
     Common base class for the event handlers.
-
-    Attributes:
-        store (synapse.storage.DataStore):
-        state_handler (synapse.state.StateHandler):
     """
 
     def __init__(self, hs):
         """
         Args:
             hs (synapse.server.HomeServer):
         """
-        self.store = hs.get_datastore()
+        self.store = hs.get_datastore()  # type: synapse.storage.DataStore
         self.auth = hs.get_auth()
         self.notifier = hs.get_notifier()
-        self.state_handler = hs.get_state_handler()
+        self.state_handler = hs.get_state_handler()  # type: synapse.state.StateHandler
         self.distributor = hs.get_distributor()
         self.clock = hs.get_clock()
         self.hs = hs

synapse/server.py

@@ -106,7 +106,7 @@
     WorkerServerNoticesSender,
 )
 from synapse.state import StateHandler, StateResolutionHandler
-from synapse.storage import DataStores, Storage
+from synapse.storage import DataStore, DataStores, Storage
 from synapse.streams.events import EventSources
 from synapse.util import Clock
 from synapse.util.distributor import Distributor
@@ -312,7 +312,7 @@ def is_mine_id(self, string):
     def get_clock(self):
         return self.clock
 
-    def get_datastore(self):
+    def get_datastore(self) -> DataStore:
         return self.datastores.main
 
     def get_datastores(self):