Get rid of deprecated cgi module.

Uses email module as suggested in docs: https://docs.python.org/3/library/cgi.html#cgi.parse_header
Fixes #1352

Author: shulcsm

src/zeep/utils.py

@@ -1,4 +1,4 @@
-import cgi
+from email.message import Message
 import inspect
 import typing
 
@@ -90,5 +90,8 @@ def detect_soap_env(envelope):
 
 def get_media_type(value):
     """Parse a HTTP content-type header and return the media-type"""
-    main_value, parameters = cgi.parse_header(value)
-    return main_value.lower()
+    msg = Message()
+    msg['content-type'] = value
+    
+    return msg.get_content_type()
+