tools (datetime, utils): fix incorrect use of naive object:

`tools.datetime.format_datetime` was expecting aware datetime objects while
`tools.utils.xmpp_date` was returning naive datetime objects. A check has been added to
the former one, and latter one has been modified to return aware datetime objects.

sat/tools/datetime.py

@@ -19,6 +19,7 @@
 # Type-check with `mypy --strict`
 # Lint with `pylint`
 
+from sat.core import exceptions
 from datetime import date, datetime, time, timezone
 import re
 from typing import Optional, Tuple
@@ -95,6 +96,7 @@ def format_datetime(
 ) -> str:
     """
     @param value: The datetime to format. Defaults to the current datetime.
+        must be an aware datetime object (timezone must be specified)
     @param include_microsecond: Include the microsecond of the datetime in the output.
     @return: The datetime formatted according to the DateTime profile specified in
         XEP-0082. The datetime is always converted to UTC before formatting to avoid
@@ -105,11 +107,14 @@ def format_datetime(
     # We format the time in UTC, since the %z formatter of strftime doesn't include colons
     # to separate hours and minutes which is required by XEP-0082. UTC allows us to put a
     # simple letter 'Z' as the time zone definition.
-    value = (
-        datetime.now(timezone.utc)
-        if value is None
-        else value.astimezone(timezone.utc)  # pylint: disable=no-member
-    )
+    if value is not None:
+        if value.tzinfo is None:
+            raise exceptions.InternalError(
+                "an aware datetime object must be used, but a naive one has been provided"
+            )
+        value = value.astimezone(timezone.utc)  # pylint: disable=no-member
+    else:
+        value = datetime.now(timezone.utc)
 
     if include_microsecond:
         return value.strftime("%Y-%m-%dT%H:%M:%S.%fZ")

sat/tools/utils.py

@@ -152,8 +152,9 @@ def xmpp_date(
     @param with_time(bool): if True include the time
     @return(unicode): XEP-0082 formatted date and time
     """
-    dtime = datetime.datetime.utcfromtimestamp(
-        time.time() if timestamp is None else timestamp
+    dtime = datetime.datetime.fromtimestamp(
+        time.time() if timestamp is None else timestamp,
+        datetime.timezone.utc
     )
 
     return format_datetime(dtime) if with_time else format_date(dtime.date())