Drop uproot3 dependency and use uproot4 for writing

[matthewfeickert]

Description

Thanks to @jpivarski's awesome and heroic work uproot v4.1.0 can now write ROOT files! 🚀

This means that once we get everything working we can drop the uproot3 dependency for the xmlio extra

pyhf/setup.py

Lines 11 to 14 in ce2ffab

	'xmlio': [
	'uproot3>=3.14.1',
	'uproot~=4.0',
	], # uproot3 required until writing to ROOT supported in uproot4

and in the tests

pyhf/.github/workflows/dependencies-head.yml

Lines 66 to 89 in ce2ffab

	uproot3:
	runs-on: ${{ matrix.os }}
	strategy:
	matrix:
	os: [ubuntu-latest]
	python-version: [3.8]
	steps:
	- uses: actions/checkout@v2
	- name: Set up Python ${{ matrix.python-version }}
	uses: actions/setup-python@v2
	with:
	python-version: ${{ matrix.python-version }}
	- name: Install dependencies
	run: |
	python -m pip install --upgrade pip setuptools wheel
	python -m pip --no-cache-dir --quiet install --upgrade --editable .[test]
	python -m pip uninstall --yes uproot3
	python -m pip install --upgrade git+git://github.com/scikit-hep/uproot3.git
	python -m pip list
	- name: Test with pytest
	run: |
	python -m pytest -r sx --ignore tests/benchmarks/ --ignore tests/contrib --ignore tests/test_notebooks.py

and in the lower bound tests

pyhf/lower-bound-requirements.txt

Lines 8 to 10 in ce2ffab

	# xmlio
	uproot3==3.14.1
	uproot==4.0.0

This shouldn't be too bad as there is only a few instances of uproot3 being used at the moment

$ git grep "uproot3" src/
src/pyhf/writexml.py:import uproot3 as uproot
src/pyhf/writexml.py:from uproot3_methods.classes import TH1

[matthewfeickert]

I've added this to the v0.6.3 board as it would be nice to get in, but if there is some thing that makes this not super easy to get in then I'll move it to v0.6.4 so that we can move forward with the release.

[matthewfeickert]

We might also need to check if this needs

pyhf/.github/workflows/lower-bound-requirements.yml

Line 30 in ce2ffab

python -m pip install --requirement lower-bound-requirements.txt

needs to be

python -m pip install --upgrade --requirement lower-bound-requirements.txt