bpo-36742: Fixes handling of pre-normalization characters in urlsplit() (GH-13017)

(cherry picked from commit d537ab0ff9767ef024f26246899728f0116b1ec3)

Co-authored-by: Steve Dower <steve.dower@python.org>
diff --git a/Lib/urllib/parse.py b/Lib/urllib/parse.py
index 1eec26e..f5b3487 100644
--- a/Lib/urllib/parse.py
+++ b/Lib/urllib/parse.py
@@ -397,13 +397,16 @@
     # looking for characters like \u2100 that expand to 'a/c'
     # IDNA uses NFKC equivalence, so normalize for this check
     import unicodedata
-    netloc2 = unicodedata.normalize('NFKC', netloc)
-    if netloc == netloc2:
+    n = netloc.rpartition('@')[2] # ignore anything to the left of '@'
+    n = n.replace(':', '')        # ignore characters already included
+    n = n.replace('#', '')        # but not the surrounding text
+    n = n.replace('?', '')
+    netloc2 = unicodedata.normalize('NFKC', n)
+    if n == netloc2:
         return
-    _, _, netloc = netloc.rpartition('@') # anything to the left of '@' is okay
     for c in '/?#@:':
         if c in netloc2:
-            raise ValueError("netloc '" + netloc2 + "' contains invalid " +
+            raise ValueError("netloc '" + netloc + "' contains invalid " +
                              "characters under NFKC normalization")
 
 def urlsplit(url, scheme='', allow_fragments=True):