watchman: refactor transport connecting to unconfuse pytype

Pytype sees `self.proc` as potentially `None` here, even though it's set by the
`_connect()` logic.  Instead of asserting, simply use the process returned by
that method (which it sets into `self.proc` before returning, so there's no
functional change here).

hgext/fsmonitor/pywatchman/__init__.py

@@ -689,9 +689,9 b' class CLIProcessTransport(Transport):'
         if self.closed:
             self.close()
             self.closed = False
-        self._connect()
-        res = self.proc.stdin.write(data)
-        self.proc.stdin.close()
+        proc = self._connect()
+        res = proc.stdin.write(data)
+        proc.stdin.close()
         self.closed = True
         return res