PEP 9999: Add PEP 508 compatibility examples

[abn]

Initial set of examples demonstrating compatibility with PEP 508.

[EpicWink]

I think this is best to be merged in once we've decided on the spec, but before the deadline (2020-09-08 19:00-07:00)

[abn]

@EpicWink I have updated the vcs examples and rebased ontop of your branch

[EpicWink]

This still has the environment marker keys, but you say in the discourse discussion that you think we should leave out the environment markers for now. I personally think the environment marker keys are good to leave in, possibly with a prefix, but the extra marker will need to be special-cased

[abn]

Ah yes, my bad. Making this change is what made me change my mind.

 
-        aiohttp = { version = ">= 3.6.1", markers = "python_version >= '3.8'" }
+        aiohttp = { version = ">= 3.6.1", python-version = ">= '3.8'" }
 

To me, the "exploded-markers" is an easy addition to what we have already. And right now the use of ">= '3.8'" seems to be like an inefficient use. For example in poetry there exists special parsing that shortens that to ">=3.8".

[abn]

@EpicWink do you want me to add the exploded form as an example as well since you already merged that?

[EpicWink]

For example in poetry there exists special parsing that shortens that to ">=3.8".

Yeah, makes sense. Right now we don't have enough time to flesh out any fancy parsing, so it makes sense to add environment marker keys down the line.

do you want me to add the exploded form as an example as well since you already merged that?

I reverted that change. I accidentally merged that in. I've marked it as deferred in the rejected-ideas section.

[EpicWink]

I've gone with the for-extra (instead of for-extras), again to ensure one line per specified requirement

[abn]

Fixed. We should mention the futre changes somewhere too eventually I guess. Making sure that it is understood that this is a first step towards better specification.