python: Convert last type comments to annotations.

We had skipped these in #14693 so we could keep generating a friendly
error on Python 3.5, but we gave that up in #19801.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

scripts/lib/zulip_tools.py

@@ -421,7 +421,7 @@ def parse_os_release() -> Dict[str, str]:
     developers, but we avoid using it, as it is not available on
     RHEL-based platforms.
     """
-    distro_info = {}  # type: Dict[str, str]
+    distro_info: Dict[str, str] = {}
     with open("/etc/os-release") as fp:
         for line in fp:
             line = line.strip()