add legacy messages (#7338)

README.rst

@@ -5,6 +5,12 @@
 
 |Build Status| |Coverage| |NumFOCUS_badge| |Binder| |Dockerhub|
 
+.. warning::
+   This is the legacy version of PyMC3, now renamed to PyMC.
+   If you are looking for the latest version of PyMC, please visit
+   `PyMC's documentation <https://www.pymc.io/welcome.html>`__
+
+
 PyMC3 is a Python package for Bayesian statistical modeling and Probabilistic Machine Learning
 focusing on advanced Markov chain Monte Carlo (MCMC) and variational inference (VI)
 algorithms. Its flexibility and extensibility make it applicable to a

pymc3/__init__.py

@@ -52,6 +52,10 @@
         handler = logging.StreamHandler()
         _log.addHandler(handler)
 
+_log.warning("The version of PyMC you are using is very outdated.\n\nPlease upgrade to the latest "
+             "version of PyMC https://www.pymc.io/projects/docs/en/stable/installation.html\n\n"
+             "Also notice that PyMC3 has been renamed to PyMC."
+             )
 
 def _check_backend_version():
     backend_paths = theano.__spec__.submodule_search_locations

pymc3/sampling.py

@@ -434,10 +434,16 @@ def sample(
             mean     sd  hdi_3%  hdi_97%
         p  0.609  0.047   0.528    0.699
     """
+    _log.info("The version of PyMC you are using is very outdated.\n\nPlease upgrade to the latest "
+                "version of PyMC https://www.pymc.io/projects/docs/en/stable/installation.html\n\n"
+                "Also notice that PyMC3 has been renamed to PyMC."
+                )
+
     # Handle deprecated/forwards-compatible kwargs
     if initvals is not None:
         start = initvals
 
+
     model = modelcontext(model)
     start = deepcopy(start)
     if start is None: