Requirement python constraint is not respected when resolving

[frostming]

• I have searched the issue tracker and believe that this is not a duplicate.

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Steps to reproduce

We have a project, that requires python>=3.6, and we define a dependency that requires python=3.8:

[tool.pdm]
python_requires = ">=3.6"

[tool.pdm.dependencies]
vendoring = {version="*", marker="python_full_version~='3.8.0'"}

Actual behavior

ResolutionImpossible

Expected behavior

The dependency gets resolved successfully, with a marker.