Switch PyMC3 to PyMC (which is the new name starting from PyMC v4.0.0)

bilby/core/sampler/__init__.py

bilby/core/sampler/pymc.py

docs/samplers.txt

...e_examples/alternative_samplers/linear_regression_pymc.py

...tive_samplers/linear_regression_pymc_custom_likelihood.py

sampler_requirements.txt

test/core/sampler/pymc_test.py

test/integration/sampler_run_test.py