Replace all use of mock with unittest.mock

containers/dockerfile-template

containers/v3-dockerfile-test-suite-python38

containers/v3-dockerfile-test-suite-python39

requirements.txt

test/core/likelihood_test.py

test/core/prior/base_test.py

test/core/prior/conditional_test.py

test/core/prior/dict_test.py

test/core/sampler/base_sampler_test.py

test/core/sampler/cpnest_test.py

test/core/sampler/dnest4_test.py

test/core/sampler/dynesty_test.py

test/core/sampler/emcee_test.py

test/core/sampler/kombine_test.py

test/core/sampler/nessai_test.py

test/core/sampler/nestle_test.py

test/core/sampler/polychord_test.py

test/core/sampler/proposal_test.py

test/core/sampler/ptemcee_test.py

test/core/sampler/pymc3_test.py