venv create & new flit build internals

summary

• you can now use a custom error message when pypyr.steps.assert raises an exception.
• new pypyr.steps.venv step to create venvs in parallel from yaml or toml config.
• new venv-create pipeline so you can provision venvs concurrently from declarative config without having to write your own pipeline or script.
• pypyr is now built & packaged by the excellent PEP517 compliant flit.

what's changed

• flake8 new version line length by @yaythomas in #286
• from setup.py to flit by @yaythomas in #290
• remove redundant dict on parser context init by @yaythomas in #292
• venv creator & log to stdout by @yaythomas in #296
• ops/build uses venv for publish verify by @yaythomas in #297

Full Changelog: v5.5.0...v5.6.0