Issue #20913: improve the SSL security considerations to first advocate using create_default_context().

Issue #21015: SSL contexts will now automatically select an elliptic curve for ECDH key exchange on OpenSSL 1.0.2 and later, and otherwise default to "prime256v1".

(should also fix a buildbot failure introduced by #20995)

Closes #20995 by Enabling better security by prioritizing ciphers
such that:

* Prefer cipher suites that offer perfect forward secrecy (DHE/ECDHE)
* Prefer ECDHE over DHE for better performance
* Prefer any AES-GCM over any AES-CBC for better performance and security
* Then Use HIGH cipher suites as a fallback
* Then Use 3DES as fallback which is secure but slow
* Finally use RC4 as a fallback which is problematic but needed for
  compatibility some times.
* Disable NULL authentication, NULL encryption, and MD5 MACs for security
  reasons

defined.

"equivalant" was caught by Tobias Käs on docs@, "seperated" and "chartruese"
were discovered by a spell-checker.

Pointed out by Colin Davis on docs@.

Issue #10141: updated new usages of AF_CAN to be in #ifdef AF_CAN rather than #ifdef HAVE_LINUX_CAN_H to allow compilation on older Linuxes.

multiprocessing.connection

- Remove configuration settings from removed _sha3.vcxproj
- Don't try to build configurations of _testembed that don't exist
  (namely, PGInstrument and PGUpdate)

base32, ascii85 and base85 codecs in the base64 module, and delay the
initialization of the unquote_to_bytes() table of the urllib.parse module, to
not waste memory if these modules are not used.

correct the fix for #20637; allow slot descriptor inheritance to take place before creating cached keys