Unconstrain pip version to be below v19.0 (!20)

As a followup to MR !17 (merged), pip 19.0.1 has been released which has a fix

c.f.:

• https://github.com/pypa/pip/issues/6158
• https://github.com/pypa/pip/releases/tag/19.0.1

This effectively reverts commit f3bb3bf2.