Improve Int and Float type stubs

[mdickinson]

This PR improves the type stubs for Int and Float, being more accurate about what's accepted for those trait types.

We need SupportsIndex, which is only available in typing from Python 3.8 onwards. So I've added typing-extensions as a dependency for traits-stubs. We can drop the dependency once we no longer support Python 3.6 and Python 3.7.

[corranwebster]

Generally looks good. A couple of questions.

[corranwebster]

Dumb question (and probably not for this PR in any case): do we have a test for this sort of thing in traits test suite?

[mdickinson]

Yes, I'm fairly sure we do. :-) I'll check.

[mdickinson]

Int:

traits/traits/tests/test_integer.py

Lines 32 to 34 in 80294cf

	class IntegerLike(object):
	def __index__(self):
	return 42

Float:

traits/traits/tests/test_float.py

Lines 21 to 44 in 80294cf

	class IntegerLike:
	def __init__(self, value):
	self._value = value
	def __index__(self):
	return self._value
	# Python versions < 3.8 don't support conversion of something with __index__
	# to float.
	try:
	float(IntegerLike(3))
	except TypeError:
	float_accepts_index = False
	else:
	float_accepts_index = True
	class MyFloat(object):
	def __init__(self, value):
	self._value = value
	def __float__(self):
	return self._value

[mdickinson]

.. which reminds me that SupportsIndex isn't actually quite right for the Float trait type with Python < 3.8. I don't think I care enough to try to distinguish. For the use-cases we care about (e.g., NumPy int types), those types implement __float__ anyway.

[corranwebster]

Do we still need _BaseInt and _BaseFloat?

[mdickinson]

They're still used in the CInt and CFloat trait types, I think. I looked at dealing with those ones too, but decided to limit the PR to avoid scope creep.