build: Begin move to uproot4

[matthewfeickert]

Description

Following PR #930 and the release of v0.5.0, this PR now moves to adopting uproot4 and its API.

As of uproot4 v4.0.0 uproot4 does not support the ability to write to ROOT files, so uproot3 must still be retained to write to ROOT. This is the only remaining requirement for uproot3 and so the move to the uproot4 API can begin now.

As uproot4 is also added to the HEAD of dependnecies workflow, the pattern of

import uproot

is used instead of

import uproot4 as uproot

as installing uproot4 from GitHub installs under the uproot namespace. So to test the HEAD of uproot4, the uproot namespace must be used now instead of waiting to drop uproot3.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Require uproot4 in the xmlio extra
   - Require release compatible with uproot v4.X series 
* Adopt the uproot4 v4.0 API
   - uproot3 still needed for writing ROOT files
* Add uproot4 to the HEAD of dependencies workflow
   - import uproot namespace everywhere uproot4 is used to allow for testing of uproot4 HEAD

[codecov]

Codecov Report

Merging #1002 (738d480) into master (5931c16) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1002   +/-   ##
=======================================
  Coverage   97.46%   97.47%           
=======================================
  Files          63       63           
  Lines        3713     3716    +3     
  Branches      524      525    +1     
=======================================
+ Hits         3619     3622    +3     
  Misses         55       55           
  Partials       39       39           

Flag	Coverage Δ
unittests	97.47% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pyhf/cli/rootio.py	92.15% <100.00%> (ø)
src/pyhf/readxml.py	94.33% <100.00%> (+0.03%)	⬆️
src/pyhf/writexml.py	95.83% <100.00%> (+0.05%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5931c16...738d480. Read the comment docs.

[jpivarski]

Just some comments inline!

[kratsg]

One thing, can you use uproot4's sphinx referencing?

intersphinx_mapping = {
    'uproot': ('https://uproot.readthedocs.io/en/latest/', None),
}

which should work pretty well in our case (unless we need to point to a specific version).