Enable auto pytest-asyncio mode

[lunkwill42]

Scope and purpose

The only async system we are using in Zino is asyncio, so all async test functions and fixtures can be assumed to be asyncio without the need for an explicit marker.

This PR enables auto-asyncio mode and removes all obsolete explicit test markers.

Contributor Checklist

Every pull request should have this checklist filled out, no matter how small it is. More information about contributing to Zino can be found in the README.

• [ ] Added a changelog fragment for towncrier
• [x] Added/amended tests for new/changed code
• [ ] Added/changed documentation
• [x] Linted/formatted the code with black, ruff and isort, easiest by using pre-commit
• [x] The first line of the commit message continues the sentence "If applied, this commit will ...", starts with a capital letter, does not end with punctuation and is 50 characters or less long. See https://cbea.ms/git-commit/
• [ ] If applicable: Created new issues if this PR does not fix the issue completely/there is further work to be done

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[github-actions[bot]]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ PYTHON	black	20		0	0.95s
✅ PYTHON	isort	20		0	0.24s
✅ PYTHON	ruff	20		0	0.02s

See detailed report in MegaLinter reports _Set VALIDATE_ALL_CODEBASE: true in mega-linter.yml to validate all sources, not only the diff_

_MegaLinter is graciously provided by _

[github-actions[bot]]

Test results

    3 files      3 suites   1m 25s :stopwatch:   573 tests   573 :white_check_mark: 0 :zzz: 0 :x: 1 671 runs  1 669 :white_check_mark: 2 :zzz: 0 :x:

Results for commit 671dad52.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.57%. Comparing base (419305a) to head (671dad5). Report is 3 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #355 +/- ## ======================================= Coverage 98.57% 98.57% ======================================= Files 77 77 Lines 9701 9701 ======================================= Hits 9562 9562 Misses 139 139 ```

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