Add empty __slots__ to Expr and Node classes.

[kwmsmith]

See issue #1268.

Consistently uses __slots__ in class hierarchies, even when empty.

[cpcloud]

Hm it'd be great to have a test. Maybe something as simple as

assert Expr.__slots__ == ()
assert Node.__slots__ == ()

[cpcloud]

When you're ready to merge please also add a tiny release note blurb to docs/source/whatsnew/0.5.0.txt

[kwmsmith]

I agree a test would be good, but it feels strange to test an internal detail like "does this class have an empty __slots__." It feels like testing the API for a private method. I don't know of any better way to test for this, though, so I'll add the asserts you mention.

[cpcloud]

Yes that test is definitely a bit awkward. If there's a way to test it using the user facing API, by all means

[kwmsmith]

When you're ready to merge please also add a tiny release note blurb to docs/source/whatsnew/0.5.0.txt

Should that be 0.8.4.txt? The latest tag I see is 0.8.3.

[cpcloud]

Sorry I meant 0.9.0. I've moved the version to a new minor version since we're dropping support for things.

[cpcloud]

@kwmsmith merge master back into your branch it should be passing now