Add an extra which specifies the lxml dependency

[sirosen]

I'm aware of #909 , which indicates that mypy wants to remove the use of lxml altogether.
And in #1001, someone was suggesting making lxml a strict requirement. I don't want that either.

I think it would be preferable, for users managing their dependencies, to state mypy[report] or similar and have that extra pull in lxml.
i.e.

# test-requirements.txt

mypy
lxml<4

is less nice to work with than

# test-requirements.txt
mypy[report]

This is really just about letting users document their requirements better by means of the extra.
Yes, you can do it with comments, but I think the extra better encodes things.

I'd rather submit as an issue than come out with a PR because I have a bunch of questions.

• Does mypy have a stance against extras? I see that the project doesn't use any today.
• Is it an issue that mypy[report] might be supported by some, but not all, versions? (I've had cases where an extra sticks around, empty, until a major version bump, because you want to maintain compatibility for install commands.)
• Is a move off of XML and XSLT imminent? (rendering this obsolete?)
• Is there a better name? mypy[coverage]? mypy[xml]?

[rnestler]

Looks like this is fixed:

mypy/setup.py

Lines 220 to 225 in 3c5f368

	extras_require={
	"dmypy": "psutil >= 4.0",
	"python2": "typed_ast >= 1.4.0, < 2",
	"reports": "lxml",
	"install-types": "pip",
	},