Fix a utcfromtimestamp warning

In python 3.12 the following DeprecationWarning
datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC.

This commit fixes it.

src/time_machine/__init__.py

@@ -351,7 +351,7 @@ def utcnow() -> dt.datetime:
     if not coordinates_stack:
         result: dt.datetime = _time_machine.original_utcnow()
         return result
-    return dt.datetime.utcfromtimestamp(time())
+    return dt.datetime.fromtimestamp(time(), dt.UTC)
 
 
 # time module

tox.ini

@@ -15,7 +15,6 @@ commands =
       -W error::ResourceWarning \
       -W error::DeprecationWarning \
       -W error::PendingDeprecationWarning \
-      -W ignore:datetime.datetime.utcfromtimestamp:DeprecationWarning \
       -W ignore:datetime.datetime.utcnow:DeprecationWarning \
       -m coverage run \
       -m pytest {posargs:tests}