bpo-39057: Fix urllib.request.proxy_bypass_environment(). (GH-17619)

Ignore leading dots and no longer ignore a trailing newline.
diff --git a/Lib/urllib/parse.py b/Lib/urllib/parse.py
index 31fd7e1..34d5f95 100644
--- a/Lib/urllib/parse.py
+++ b/Lib/urllib/parse.py
@@ -1056,9 +1056,9 @@
     """splitport('host:port') --> 'host', 'port'."""
     global _portprog
     if _portprog is None:
-        _portprog = re.compile('(.*):([0-9]*)$', re.DOTALL)
+        _portprog = re.compile('(.*):([0-9]*)', re.DOTALL)
 
-    match = _portprog.match(host)
+    match = _portprog.fullmatch(host)
     if match:
         host, port = match.groups()
         if port: