Fix deprecation of "datetime.utcfromoffset()"

Although it's not recommended, we still use a naive datetime for UTC
time (to calculate the local offset). I'm sure there's a more elegant
solution, but as this is code for old, untestable platforms, I'd
rather not radically change the implementation.

loguru/_datetime.py

@@ -94,7 +94,9 @@ def aware_now():
         seconds = local.tm_gmtoff
         zone = local.tm_zone
     except AttributeError:
-        offset = datetime_.fromtimestamp(timestamp) - datetime_.utcfromtimestamp(timestamp)
+        # Workaround for Python 3.5.
+        utc_naive = datetime_.fromtimestamp(timestamp, tz=timezone.utc).replace(tzinfo=None)
+        offset = datetime_.fromtimestamp(timestamp) - utc_naive
         seconds = offset.total_seconds()
         zone = strftime("%Z")