Drop direct dependency on iso8601

iso8601.iso8601.UTC has been equivalent to datetime.timezone.utc in
Python 3. Because python 2 is no longer supported, we can directly use
the built-in implementation.

Also replace iso8601.parse_date by the common function in oslo.utils .

Change-Id: I933cc5dd1fa76e320cd96cd8e9b9a7963ff70375

requirements.txt

@@ -19,7 +19,6 @@ PrettyTable>=0.7.1 # BSD
 alembic>=1.5.0 # MIT
 netaddr>=0.7.18 # BSD
 paramiko>=2.7.1 # LGPLv2.1+
-iso8601>=0.1.11 # MIT
 jsonschema>=4.0.0 # MIT
 python-cinderclient>=4.0.1 # Apache-2.0
 keystoneauth1>=3.16.0 # Apache-2.0