Fix flake8 issues in tests and docs

.gitlab-ci.yml

.pre-commit-config.yaml

CONTRIBUTING.md

docs/conf.py

setup.cfg

test/calibration_test.py

test/context.py

test/conversion_test.py

test/cosmology_test.py

test/detector_test.py

test/example_test.py

test/grid_test.py

test/gw_example_test.py

test/gw_likelihood_test.py

test/gw_plot_test.py

test/gw_prior_test.py

test/gw_result_test.py

test/gw_source_test.py

test/gw_utils_test.py

test/hyper_pe_test.py