chore: Use _version.py to follow setuptools_scm recommendations

[matthewfeickert]

Description

Following pypa/setuptools-scm#576 change the generated version file (c.f. PR #1450) to be named _version.py

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* Rename generated version file from version.py to _version.py following setuptools_scm recommendations
   - c.f. https://github.com/pypa/setuptools_scm/pull/576
   - Avoids publishing the contents of "version.py" as externally visible API
* Amends PR #1450

[codecov]

Codecov Report

Merging #1480 (ba6996d) into master (2272410) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1480   +/-   ##
=======================================
  Coverage   97.65%   97.65%           
=======================================
  Files          63       63           
  Lines        4006     4006           
  Branches      565      565           
=======================================
  Hits         3912     3912           
  Misses         55       55           
  Partials       39       39           

Flag	Coverage Δ
contrib	25.51% <100.00%> (ø)
unittests	97.42% <100.00%> (ø)

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

Impacted Files	Coverage Δ
src/pyhf/__init__.py	98.64% <100.00%> (ø)

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[matthewfeickert]

@kratsg I'm just going to merge this to move forward (I know that you're circumstantially extra busy this week) but please make any comments if you'd like something changed. I'm happy to revert or make a fix PR.