v0.4.0

This is a minor release from v0.3.4 → v0.4.0.

Changes:
  - feat: Drop Python 2.7 support (#704)
  - docs: Use pip best practices with python -m pip (#730)
  - fix: Public release canary should be using [backends] option (#728)
  - fix: iminuit optimiser is never verbose (#724)