linkcheck: Use correct function to convert from UTC time to UNIX epoch

After subtracting the offset, we get time in UTC. But time.mktime()
expects a time struct in local time, not in UTC. The correct function
for converting UTC time to UNIX epoch is calendar.timegm().

This fixes hanging tests when the local timezone is to the west of UTC.
One could test this using:

TZ=America/New_York python3 -m pytest -k test_too_many_requests_retry_after_HTTP_date

sphinx/builders/linkcheck.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import calendar
 import contextlib
 import json
 import re
@@ -491,7 +492,7 @@ def limit_rate(self, response_url: str, retry_after: str) -> float | None:
                     parsed = parsedate_tz(retry_after)
                     assert parsed is not None
                     # the 10th element is the GMT offset in seconds
-                    next_check = time.mktime(parsed[:9]) - (parsed[9] or 0)
+                    next_check = float(calendar.timegm(parsed[:9])) - (parsed[9] or 0)
                 except (AssertionError, TypeError, ValueError):
                     # TypeError: Invalid date format.
                     # ValueError: Invalid date, e.g. Oct 52th.