chore: remove attrs from requirements-test.txt

It used to be necessary to fix the version of attrs when using pytest. This problem has been fixed now, so I removed it.
https://stackoverflow.com/a/58198754

requirements-tests.txt

 -r requirements.txt
-attrs==20.3.0
 pytest==6.2.1
 pytest-cov==2.10.1
 flake8==3.8.4