Use optional dependencies for gmpy2 and fastcache

[asmeurer]

gmpy and fastcache should be optional dependencies. I believe these are supported by conda now. It should make it so that they get installed with sympy, but they can be conda removed without also removing sympy.

Also I don't know why setuptools is a run dependency of sympy. It shouldn't be needed.

[ocefpaf]

Also I don't know why setuptools is a run dependency of sympy. It shouldn't be needed.

#13 (comment)

[asmeurer]

We should look into that. The conda entry points ought to work just fine on Windows.

[isuruf]

There's no support for optional dependencies in conda. There's run_constrained which says that if a package is installed in the environment, the version has to satisfy the constraint. i.e. we can say sympy needs gmpy2>=2.0.8 if gmpy2 is installed, but installing sympy will not install gmpy2.

[asmeurer]

I guess I misunderstood the feature. I'd like something where it tries to install the dependency if it can, but if it isn't available or satisfiable to install it, it just skips it.