Change scope of test_constants.py

[honno]

This PR:

• Removes dependencies on most xp functions, as they're not interesting IMO so unnecessarily increase dependencies
  • Note these tests were written before Clarify that constants are Python scalars array-api#169 where such use of arrays was necessary
  • Opted to use pure Python equalities instead
• Instance checks the constants should work like floats i.e. supports __float__
  • float check would hurt duck typed implementations, see Consider removing array constants array-api#154 (comment)
  • numerary stuff doesn't look necessary here (interesting for other parts of the suite tho)
• Adds interesting equality checks that @pmeier wrote for explicitly check for Python scalars in constant tests #15... useful if we're not assuming float

Clarifying we don't need constants to be specifically float should close #14 and #15.

[asmeurer]

Shouldn't this be xp.e and xp.pi? Although why not just parameterize name and lookup getattr(xp, name) in the test?

[honno]

Ah no, coz I'm using Python's math just to get the rough approximations (i.e. to test the xp constants are at least in-between the floor and ceil of the respective math constants).

[asmeurer]

Just a few comments. Looks good otherwise.