Getting rid of core.compatibility

[oscargus]

core.compatibility was used to support both Python 2 and 3. However, it was also the location for some other support functions. Since only Python 3 is now supported, the remaining functions are moved to more suitable locations.

These are:

• as_int => utilities.misc
• is_sequence => utilities.iterables
• iterable => utilities.iterables
• ordered => core.sorting
• default_sort_key => core.sorting

[ThePauliPrinciple]

Since these functions are more generic than providing compatibility, should these still be available in core.compatibility for some time?

[oscargus]

Yes, it seems to be the policy to keep a deprecated stub, calling the correct location. To make it even more confusing, ordered and default_sort_key had their import location (although not heavily used) in utilities.iterables.

[asmeurer]

Right now there are deprecations on each individual function, but would it be better to completely deprecate the submodule, so that just importing it raises the deprecation warning? The plan is to completely delete the file.

[asmeurer]

I am updating this to deprecate the entire submodule in #22900. ordered and default_sort_key are in the top-level namespace and should be imported directly, like from sympy import ordered, default_sort_key. The remaining helper functions are only designed for used internal use and should not be imported by user code.