Update pyjwt version to >=2.0.0,<3.0.0

Pyjwt recently made a 2.0 release. This release is not compatible with
the 1.x series code. We fix the registry to be compat with pyjwt 2.x and
then exclude 1.x.

More details here:
  https://pyjwt.readthedocs.io/en/stable/changelog.html#jwt-encode-return-type

Change-Id: I5c8e6fa0bb593efc70d57413b6d3274883a5a9b5

requirements.txt

@@ -7,4 +7,4 @@ requests
 openstacksdk
 python-dateutil
 rehash
-pyjwt
+pyjwt>=2.0.0,<3.0.0

zuul_registry/main.py

@@ -128,8 +128,7 @@ class Authorization(cherrypy.Tool):
             else:
                 self.unauthorized()
         self.log.debug('Generate %s token', level)
-        token = jwt.encode({'level': level}, 'secret',
-                           algorithm='HS256').decode('utf8')
+        token = jwt.encode({'level': level}, 'secret', algorithm='HS256')
         return {'token': token,
                 'access_token': token}