resolve sonarcloud warnings

[christianhelle]

• use isinstance() for direct type comparison
• Specify return types
• Use assertIn instead of assertTrue

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 97.96%. Comparing base (000f7e7) to head (abb1dd2). Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
src/autofaker/generator.py	0.00%	1 Missing :warning:
tests/test_create_anonymous_builtins.py	94.73%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #49 +/- ## ========================================== - Coverage 97.96% 97.96% -0.01% ========================================== Files 46 46 Lines 2508 2506 -2 ========================================== - Hits 2457 2455 -2 Misses 51 51 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[sonarcloud[bot]]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarCloud