Fix usage of deprecated .best_match()

The webob emits a warning for best_match method for the reason that its
algorithm does not conform to RFC 7231 [1]. This patch replace
`best_match` with `acceptable_offers` as v2 API does [2].

The following deprecation warnings are gone with the patch:

- cceptparse.py:1047: DeprecationWarning: The behavior of
  AcceptValidHeader.best_match is currently being maintained for
  backward compatibility, but it will be deprecated in the future, as it
  does not conform to the RFC.

[1]
https://docs.pylonsproject.org/projects/webob/en/stable/api/webob.html#webob.acceptparse.AcceptValidHeader.best_match
[2]
https://github.com/openstack/tacker/blob/master/tacker/sol_refactored/api/wsgi.py#L37

Change-Id: Ia68fa8acb7a27c17f9da7eea587112da4dedb892
Closes-Bug: #1976220
(cherry picked from commit e8dfc6b037)

tacker/wsgi.py

@@ -357,11 +357,11 @@ class Request(webob.Request):
         type_from_header = self.get_content_type()
         if type_from_header:
             return type_from_header
-        ctypes = ['application/json', 'text/plain', 'application/zip']
+        offers = ['application/json', 'text/plain', 'application/zip']
 
         # Finally search in Accept-* headers
-        bm = self.accept.best_match(ctypes)
-        return bm or 'application/json'
+        ctypes = self.accept.acceptable_offers(offers)
+        return ctypes[0][0] if ctypes else 'application/json'
 
     def get_content_type(self):
         allowed_types = ("application/json", "application/zip")