Resolve #44 - `manifest_url` for SIP labels

[nutjob4life]

Summary

• Resolve #44 …
  • Test case to expose the issue (http versus file URL)
  • New constant for SIP manifest locations
  • Slight PEP-8 cosmetic fix (spaces around +)

Test Data and/or Report

$ bin/test
Running tests at level 1
Running zope.testrunner.layer.UnitTests tests:
  Set up zope.testrunner.layer.UnitTests in 0.000 seconds.
  Running:
    3/15 (20.0%) test_logging_arguments (pds.aipgen.tests.test_utils.ArgumentTestCase)usage: 

  Ran 15 tests with 0 failures, 0 errors, 0 skipped in 0.046 seconds.
Tearing down left over layers:
  Tear down zope.testrunner.layer.UnitTests in 0.000 seconds.

Related Issues

• (Future issue number) For when "they" realize that manifest_url maybe isn't the right idea 🤷‍♀️