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.

.azure-pipelines/azure-pipelines.yml

@@ -145,8 +145,6 @@ stages:
         parameters:
           testFormat: devel/linux-community/{0}
           targets:
-            - name: Debian Bullseye
-              test: debian-bullseye/3.9
             - name: Debian Bookworm
               test: debian-bookworm/3.11
             - name: ArchLinux