requirements: add missing pytz package

`pytz` package was included by oslo.log dependency on
oslo.serialization.
Recently, this commit removed the pytz package for
Python versions >= 3.9:
openstack/oslo.serialization@9aaea36

Which means `pytz` needs to be added as a requirement to
be able to support the current mainline Python 3.11.

Change-Id: I3dadbe0ead2369565afd7b7feed7761ff4a7db57
Signed-off-by: Adrian Vladu <avladu@cloudbasesolutions.com>

requirements.txt

@@ -8,6 +8,7 @@ six>=1.7.0
 Babel>=1.3
 oauthlib
 netifaces
+pytz
 PyYAML
 requests
 untangle==1.2.1