user_agent: Compile the regular expression.

We use this single regular expression for processing essentially every
request, so it's definitely worth hinting to Python that we're going
to do so by compiling it.  Saves about 40us per request.

zerver/lib/user_agent.py

@@ -4,11 +4,13 @@ from typing import Dict
 # Warning: If you change this parsing, please test using
 #   zerver/tests/test_decorators.py
 # And extend zerver/tests/fixtures/user_agents_unique with any new test cases
+pattern = re.compile(
+    """^ (?P<name> [^/ ]* [^0-9/(]* )
+    (/ (?P<version> [^/ ]* ))?
+    ([ /] .*)?
+    $""", re.X)
+
 def parse_user_agent(user_agent: str) -> Dict[str, str]:
-    match = re.match(
-        """^ (?P<name> [^/ ]* [^0-9/(]* )
-             (/ (?P<version> [^/ ]* ))?
-             ([ /] .*)?
-           $""", user_agent, re.X)
+    match = pattern.match(user_agent)
     assert match is not None
     return match.groupdict()