Consolidate all warnings.simplefilter calls to sympy.utilities.exceptions

This is the file where SymPyDeprecationWarning is defined, so the filter will
always be set when that warning is used.

This also changes the warning to once, rather than always, which matches the
default Python warning behavior.

This does cause the SymPyDeprecationWarning to show, which by default it
wouldn't, because Python hides DeprecationWarnings by default.  This is
intentional, as we want our users to know about deprecated behavior.