Merge: #21991: make headerregistry params property MappingProxyType.

diff --cc Lib/email/headerregistry.py
Simple merge

diff --cc Misc/NEWS
index 1aca6e7dff59a3047d32a40daa12ff02e16996bf,e04f4cc02d3b08a4ae4ff30ede9dc26a343acd3e..4316b75f5816a440c66a24a30b47319c48d52da3
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -181,9 -33,10 +181,13 @@@ Core and Builtin
  Library
  -------
  
+ - Issue #21991: Make email.headerregistry's header 'params' attributes
+   be read-only (MappingProxyType).  Previously the dictionary was modifiable
+   but a new one was created on each access of the attribute.
+ 
 +- Issue #22638: SSLv3 is now disabled throughout the standard library.
 +  It can still be enabled by instantiating a SSLContext manually.
 +
  - Issue #22641: In asyncio, the default SSL context for client connections
    is now created using ssl.create_default_context(), for stronger security.