Update vendored dependencies

[techalchemy]

• Updated requirementslib
• Fix local relative path bug whici caused paths to be output as
  absolute (test added to requirementslib)
• Fix SSH normalization bug (test added to requirementslib)
• Fixes Absolute paths for -e . in lock -r #2440, Relative git ssh module having ssh:// appended #2441

Signed-off-by: Dan Ryan dan@danryan.co

[frostming]

Consider adding a note how to patch the vendors in contrubution guide? Also invoke is not present in Pipfile

[techalchemy]

@frostming do you mind opening an issue about the patching issue? it's a bit complicated which is why i didn't do it yet (also I'm just bad at keeping up with docs)