Improve Channel object support for user channels

* Adds support for additional channel URL patterns

Author: JarbasAl

pytube/extract.py

@@ -187,6 +187,8 @@ def channel_name(url: str) -> str:
 
     - :samp:`https://youtube.com/c/{channel_name}/*`
     - :samp:`https://youtube.com/channel/{channel_id}/*
+    - :samp:`https://youtube.com/u/{channel_name}/*`
+    - :samp:`https://youtube.com/user/{channel_id}/*
 
     :param str url:
         A YouTube url containing a channel name.
@@ -196,7 +198,9 @@ def channel_name(url: str) -> str:
     """
     patterns = [
         r"(?:\/(c)\/([\d\w_\-]+)(\/.*)?)",
-        r"(?:\/(channel)\/([\w\d_\-]+)(\/.*)?)"
+        r"(?:\/(channel)\/([\w\d_\-]+)(\/.*)?)",
+        r"(?:\/(u)\/([\d\w_\-]+)(\/.*)?)",
+        r"(?:\/(user)\/([\w\d_\-]+)(\/.*)?)"
     ]
     for pattern in patterns:
         regex = re.compile(pattern)