Disable installing build-harness venv via python 2

[charlieparkes]

• Use virtualenv to install bh/venv in python 2
• Disable installing bh/venv via python 2 by default, but leave the feature to be manually enabled in the future

[charlieparkes]

Note: If you must develop in Python 2, install the libs via your project's dev-dependencies, you can write a target that offloads some complexity to the build-harness.

lint: pipenv reports/
    @$(MAKE) python/lint/announce/isort
    @$(PIPENV_RUN) isort $(ISORT_FLAGS) --check-only $(PYTHON_LINT_TARGETS) --diff
    @$(MAKE) python/lint/announce/flake8
    @$(PIPENV_RUN) flake8 $(FLAKE8_FLAGS) $(PYTHON_LINT_TARGETS)
.PHONY: lint

In this example, build-harness provides:

• Targets pipenv, reports/, and python/lint/announce/...
• Variables PIPENV_RUN, PYTHON_LINT_TARGETS, ISORT_FLAGS, and FLAKE8_FLAGS

You could simplify this even more, but this is an example replicating the exact functionality build-harness provides for isort and flake8. Check the source for other examples you may draw inspiration from if you decide to write a custom target.

[charlieparkes]

@chavlin I'd prefer not to add it to the readme, as it's not "officially supported" by the project. In the unlikely event someone brings this up, refer them to this PR. (it is the github way)

[charlieparkes]

We just discovered that this breaks our jenkins builds. It's almost like running all builds on the same python 2 binary is a bad idea. 🙄 Will need to find a workaround for CI systems that don't run builds on the project's desired version of python.

[charlieparkes]

Ok, updated to work in python 2 build environments.

1. If project is using pipenv, set python_requires in .build-harness.Pipfile
2. bh/venv will now run with warning when local python is python 2, but the project is python 3, as python 2 pipenv will let us run python 3 packages and redirect to the correct system version of python, if it's installed