pipenv integration

[Viktor Chaptsev]

pipenv is great:

Enables truly deterministic builds, while easily specifying only what you want.
Generates and checks file hashes for locked dependencies.
Automatically install required Pythons, if pyenv is available.
Automatically finds your project home, recursively, by looking for a Pipfile.
Automatically generates a Pipfile, if one doesn't exist.
Automatically creates a virtualenv in a standard location.
Automatically adds/removes packages to a Pipfile when they are un/installed.
Automatically loads .env files, if they exist.

Progress:

• install pipenv
• use pipenv install --system
• split requirements (base/dev)
• add pipenv check to CI
• initialize Pipfile and Pipfile.lock
• update requirements
• test it