The tests for FileDependency and DirectoryDependency use the following assert:

assert dep.to_pep_508() == pep_508_output or pep_508_input

which is equal to

assert (dep.to_pep_508() == pep_508_output) or pep_508_input

and thus, always true if there is an pep_508_input. Actually it should be

assert dep.to_pep_508() == (pep_508_output or pep_508_input)

While fixing the tests, I noticed that the output for relative paths was platform-dependent. I changed it to always return posix paths.

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