test(scripts): run pytest tests individually

Running tests concurrently can cause temporary files, such as the file
used to backup the commit message, to be deleted by one test but
accessed by another test later on.

scripts/test

@@ -4,7 +4,7 @@ set -e
 export PREFIX='poetry run python -m '
 export REGEX='^(?![.]|venv).*'
 
-${PREFIX}pytest -n 3 --cov-report term-missing --cov-report=xml:coverage.xml --cov=commitizen tests/
+${PREFIX}pytest --cov-report term-missing --cov-report=xml:coverage.xml --cov=commitizen tests/
 ${PREFIX}black commitizen tests --check
 ${PREFIX}isort --check-only commitizen tests
 ${PREFIX}flake8 commitizen/ tests/