Avoid module level sympy yet another time (#5576)
Avoid module level sympy yet another time (#5576)

Just as has been demonstrated many times in the past we should not be
importing sympy at the module level. In #5358 another import was added
and was immediately flagged as an import speed regression [1]. Sympy is
a very heavy dependency and is slow to import (and slow to run too).
Adding a sympy import to the module level makes and is honestly a
dependency we should be reluctant to add anywhere because it becomes
the primary bottleneck whenever it is used. This commit removes the top
level sympy import and moves to be a runtime import in the single method
that uses sympy. Just as it was done in #5416, #5149, #4385, and #3156
before.

Fixes #5575

[1] https://qiskit.github.io/qiskit/#import.QiskitImport.time_qiskit_import?commits=300e9507
QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py