Remove support for cryptography < 3.4 (#878)

* Stop passing backend to cryptography.

* Make public_bytes() fallback the default.

* Remove compatibility code for older cryptography versions.

* Require cryptography 3.4+.

* Restrict to cryptography >= 3.4 in integration tests.

* Remove Debian Bullseye from CI.

It only supports cryptography 3.3.

* Improve imports.

* Remove no longer existing conditional.

plugins/modules/openssl_privatekey_info.py

@@ -17,7 +17,7 @@ description:
     V(none) is returned for RV(key_is_consistent).
   - It uses the cryptography python library to interact with OpenSSL.
 requirements:
-  - cryptography >= 1.2.3
+  - cryptography >= 3.4
 author:
   - Felix Fontein (@felixfontein)
   - Yanis Guenane (@Spredzy)