Jenkinsfile: Remove support for Jenkins ...

The Linux build is broken because containers don't have root access in the container.
The Windows build is broken because my tox install is not in the system PATH.

These are definitely fixable, but I don't have the energy to go through the
winding maze of Jenkins documentation, the web interface and get told all my
plugins are orphaned and full of security holes. It's too much.

So I'm just dropping Jenkins support for now.

Jenkinsfile: Try to fix Linux build

env: Dump environment system

Jenkinsfile: Fix build

build: Switch to regular Python building

env: Print mutmut results on error on Linux

Jenkinsfile: Only cleanup on success

Jenkinsfile: Run domut_ci before dotest_ci ...

domut_ci does light testing first and should catch errors earlier

env.ps1: Add domut_ci command

env.sh: Add CI-friendly domut_ci command

Jenkinsfile: Add new file used for CI ...

Jenkins currently builds on my Windows 10 Home machine and an Alpine
Python compiler.
This is my first working attempt at having a CI file.