On deprecating code

[Aaron Meurer]

In the process of doing the release, I've noticed that a lot of
deprecations done in the code are not fully following our deprecation
policy https://github.com/sympy/sympy/wiki/Deprecating-policy. Please
read this policy if you do any deprecations.

In particular, two things:

- Every deprecation should have an associated "deprecation removal"
issue, which is tagged in the SymPyDeprecationWarning. This serves two
purposes: to give users a place to see why the behavior was deprecated
and how to fix their code, and to track the eventual removal of the
deprecated behavior.

- Deprecations should not be done lightly. If every time a users
upgrade SymPy they sees tons of deprecation warnings and have to do a
bunch of work to change their code around, then eventually they will
get annoyed and stop updating SymPy (or stop using it altogether).

Aaron Meurer