Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code.

This PR fixes the propagation of environment markers for dependencies. Before this change, the environment markers where not properly propagated which could cause false reports of incompatibilities. For instance, if you had the black dependency with the python_version >= '3.6' and implementation_name != 'pypy' environment marker, the resolution would fail with the following message:

[SolverProblemError]
The current project's Python requirement (~2.7 || ^3.4) is not compatible with some of the required packages Python requirement:
  - black requires Python >=3.6

Because no versions of black match >19.10b0,<20.0
 and black (19.10b0) requires Python >=3.6, black is forbidden.
So, because my-package depends on black (^19.10b0), version solving failed.

This is obviously wrong since we specified python_version >= '3.6' but the marker was lost during the resolution.

I took this opportunity to improve the way we handle markers to avoid producing markers that are obviously wrong or that can't be satisfied.