Log Message:
py-pip: updated to 19.1.1

19.1.1:

Features
- Restore pyproject.toml handling to how it was with pip 19.0.3 to prevent
  the need to add --no-use-pep517 when installing in editable mode.

Bug Fixes
- Fix a regression that caused @ to be quoted in pypiserver links.
  This interfered with parsing the revision string from VCS urls.