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

This MR updates the PyMC3 interface to use the new version of PyMC, which is now v4.0.0 (the version number suffix has been dropped). This is mainly a very straightforward port by replacing imports of pymc3 with pymc. The Theano library has also been renamed to aesara, so those are changed too.

The test suite runs and also the two examples, linear_regression_pymc.py and linear_regression_pymc_custom_likelihood.py, both work exactly as expected.

bilby/core/sampler/base_sampler.py

@@ -169,6 +169,10 @@ class Sampler(object):
         System exit code to return on interrupt
     kwargs: dict
         Dictionary of keyword arguments that can be used in the external sampler
+    hard_exit: bool
+        Whether the implemented sampler exits hard (:code:`os._exit` rather
+        than :code:`sys.exit`). The latter can be escaped as :code:`SystemExit`.
+        The former cannot.
 
     Raises
     ======
@@ -185,6 +189,7 @@ class Sampler(object):
 
     default_kwargs = dict()
     npool_equiv_kwargs = ["queue_size", "threads", "nthreads", "npool", "cores"]
+    hard_exit = False
 
     def __init__(
         self,
@@ -681,12 +686,19 @@ class Sampler(object):
         """
         Make sure that if a pool of jobs is running only the parent tries to
         checkpoint and exit. Only the parent has a 'pool' attribute.
+
+        For samplers that must hard exit (typically due to non-Python process)
+        use :code:`os._exit` that cannot be excepted. Other samplers exiting
+        can be caught as a :code:`SystemExit`.
         """
         if self.npool == 1 or getattr(self, "pool", None) is not None:
             self._log_interruption(signum=signum)
             self.write_current_state()
             self._close_pool()
-            sys.exit(self.exit_code)
+            if self.hard_exit:
+                os._exit(self.exit_code)
+            else:
+                sys.exit(self.exit_code)
 
     def _close_pool(self):
         if getattr(self, "pool", None) is not None: