add aurweb.time module

This module includes timezone-based utilities for a FastAPI request.
This commit introduces use of the AURTZ cookie within get_request_timezone.
This cookie should be set to the user or session's timezone.

* `make_context` has been modified to parse the request's timezone
  and include the "timezone" and "timezones" variables, along with
  a timezone specified "now" date.
+ Added `Timezone` attribute to aurweb.testing.requests.Request.user.

Signed-off-by: Kevin Morris <kevr@0cost.org>

aurweb/time.py

@@ -0,0 +1,63 @@
+import zoneinfo
+
+from collections import OrderedDict
+from datetime import datetime
+
+from fastapi import Request
+
+import aurweb.config
+
+
+def tz_offset(name: str):
+    """ Get a timezone offset in the form "+00:00" by its name.
+
+    Example: tz_offset('America/Los_Angeles')
+
+    :param name: Timezone name
+    :return: UTC offset in the form "+00:00"
+    """
+    dt = datetime.now(tz=zoneinfo.ZoneInfo(name))
+
+    # Our offset in hours.
+    offset = dt.utcoffset().total_seconds() / 60 / 60
+
+    # Prefix the offset string with a - or +.
+    offset_string = '-' if offset < 0 else '+'
+
+    # Remove any negativity from the offset. We want a good offset. :)
+    offset = abs(offset)
+
+    # Truncate the floating point digits, giving the hours.
+    hours = int(offset)
+
+    # Subtract hours from the offset, and multiply the remaining fraction
+    # (0 - 0.99[repeated]) with 60 minutes to get the number of minutes
+    # remaining in the hour.
+    minutes = int((offset - hours) * 60)
+
+    # Pad the hours and minutes by two places.
+    offset_string += "{:0>2}:{:0>2}".format(hours, minutes)
+    return offset_string
+
+
+SUPPORTED_TIMEZONES = OrderedDict({
+    # Flatten out the list of tuples into an OrderedDict.
+    timezone: offset for timezone, offset in sorted([
+        # Comprehend a list of tuples (timezone, offset display string)
+        # and sort them by (offset, timezone).
+        (tz, "(UTC%s) %s" % (tz_offset(tz), tz))
+        for tz in zoneinfo.available_timezones()
+    ], key=lambda element: (tz_offset(element[0]), element[0]))
+})
+
+
+def get_request_timezone(request: Request):
+    """ Get a request's timezone by its AURTZ cookie. We use the
+    configuration's [options] default_timezone otherwise.
+
+    @param request FastAPI request
+    """
+    if request.user.is_authenticated():
+        return request.user.Timezone
+    default_tz = aurweb.config.get("options", "default_timezone")
+    return request.cookies.get("AURTZ", default_tz)