Issue #26586: Merge excessive HTTP header handling from 3.5

diff --cc Lib/http/server.py
Simple merge

diff --cc Lib/test/test_httpservers.py
Simple merge

diff --cc Misc/NEWS
index 8a7fac4ebdbd0aa6738cd96b1d8e7d823e79af88,9682b409d1415bdc030eb14976cdb7889dce2d0d..cba8466109bdd1ccf4fcdc062f26d67c8e6fbbeb
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -237,8 -98,11 +237,12 @@@ Core and Builtin
  Library
  -------
  
+ - Issue #26586: In http.server, respond with "413 Request header fields too
+   large" if there are too many header fields to parse, rather than killing
+   the connection and raising an unhandled exception.  Patch by Xiang Zhang.
+ 
 +- Issue #26676: Added missing XMLPullParser to ElementTree.__all__.
 +
  - Issue #22854: Change BufferedReader.writable() and
    BufferedWriter.readable() to always return False.