tests: change Python code style to pass ruff checks

Most of the changes consisted of removing unused imports and unnecessary
f-strings.

tests: improve test_17_07_ssl_ciphers

Change TLS proto version on the test httpd server to test setting
combinations of --tls13-ciphers and --ciphers.

To not let the changed con

tests: improve test_17_07_ssl_ciphers

Change TLS proto version on the test httpd server to test setting
combinations of --tls13-ciphers and --ciphers.

To not let the changed config of the httpd server bleed into the next
test, clean and reload on each test. Because a reload is slow, only
do this if the config is different than the loaded config. For this
the httpd.reload_if_config_changed() method is added.

Overloading of autouse fixtures does not seem to work. For the test
httpd server to be reloaded with a clean config in test_18_methods,
to not be affected by the config changes in test_17_ssl_use, the two
class scope fixtures of test_18_methods are now combined.

Closes #14589

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tidy-up: whitespace [ci skip]

pytest: add DELETE tests, check server version

- add tests for DELETE working
- check apache version in keepalive test
- fix some comments

Closes #13679