Issue #27938: Add a fast-path for us-ascii encoding

Issue #27938: Add a fast-path for us-ascii encoding

Other changes:

* Rewrite _Py_normalize_encoding() as a C implementation of
  encodings.normalize_encoding(). For example, " utf-8 " is now normalized to
  "utf_8". So the fast path is now used for more name variants of the same
  encoding.
* Avoid strcpy() when encoding is NULL: call directly the UTF-8 codec