Free sympy version

[mpharrigan]

Following #1167, we restrict sympy for mypy type checking issues. Need to investigate

[mpharrigan]

The issues seem to stem from mypy treating later versions of sympy.Expr as typing.Any -- this seems to be particularly accute when combined with the TypeIs annotation in is_symbolic. I found this by using typing_extensions.reveal_type

[mpharrigan]

Note that sympy does not have a typing marker, so mypy ignores it (per our mypy.ini). If I use

[mypy-sympy.*]
follow_untyped_imports = True

to force it to process sympy (whose code base does appear to have type annotations, see the potentially relevent PR sympy/sympy#27350) sympy.Expr gets kept -- but mypy flags several places where we're being fast-and-loose with sympy.Basic vs sympy.Expr etc