maint: stop using deprecated utcnow

Potentially it might be addressing
- gabrielfalcao#474

In our case we got datalad tests failing since we trigger errors on all new
deprecation warnigns and this was the one this time

    .pybuild/cpython3_3.12_datalad/build/datalad/downloaders/tests/test_http.py: 13 warnings
      /usr/lib/python3/dist-packages/httpretty/core.py:1077: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
        now = datetime.utcnow()

httpretty/core.py

@@ -68,8 +68,7 @@
 
 from .errors import HTTPrettyError, UnmockedError
 
-from datetime import datetime
-from datetime import timedelta
+from datetime import (datetime, timedelta, UTC)
 from errno import EAGAIN
 
 old_socket = socket.socket
@@ -611,7 +610,7 @@ def normalize_headers(self, headers):
         return new
 
     def fill_filekind(self, fk):
-        now = datetime.utcnow()
+        now = datetime.now(UTC)
 
         headers = {
             'status': self.status,