Add support for Python 3.8

[bdice]

Description

This PR tests signac on Python 3.8. I am not aware of any other configuration needed to officially support Python 3.8.

Motivation and Context

Resolves #254.

Types of Changes

• Documentation update
• Bug fix
• New feature
• Breaking change¹

¹The change breaks (or has the potential to break) existing functionality.

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My name is on the list of contributors.
• My code follows the code style guideline of this project.
• The changes introduced by this pull request are covered by existing or newly introduced tests.

If necessary:

• I have updated the API documentation as part of the package doc-strings.
• I have created a separate pull request to update the framework documentation on signac-docs and linked it here.
• I have updated the changelog and added all related issue and pull request numbers for future reference (if applicable). See example below.

Example for a changelog entry: Fix issue with launching rockets to the moon (#101, #212).

[bdice]

There is a problem with running codecov on Python 3.8. For now, I'm only running codecov on Python < 3.8.

Failed test log: https://circleci.com/gh/glotzerlab/signac/4308

Traceback (most recent call last):
  File "/usr/local/lib/python3.8/runpy.py", line 183, in _run_module_as_main
    mod_name, mod_spec, code = _get_module_details(mod_name, _Error)
  File "/usr/local/lib/python3.8/runpy.py", line 142, in _get_module_details
    return _get_module_details(pkg_main_name, error)
  File "/usr/local/lib/python3.8/runpy.py", line 109, in _get_module_details
    __import__(pkg_name)
  File "/home/circleci/repo/venv/lib/python3.8/site-packages/coverage/__init__.py", line 13, in <module>
    from coverage.control import Coverage, process_startup
  File "/home/circleci/repo/venv/lib/python3.8/site-packages/coverage/control.py", line 29, in <module>
    from coverage.python import PythonFileReporter
  File "/home/circleci/repo/venv/lib/python3.8/site-packages/coverage/python.py", line 11, in <module>
    from coverage.parser import PythonParser
  File "/home/circleci/repo/venv/lib/python3.8/site-packages/coverage/parser.py", line 320, in <module>
    OP_BREAK_LOOP = _opcode('BREAK_LOOP')
  File "/home/circleci/repo/venv/lib/python3.8/site-packages/coverage/parser.py", line 278, in _opcode
    return dis.opmap[name]
KeyError: 'BREAK_LOOP'

Potentially related issue: nedbat/coveragepy#772

[b-butler]

If we plan on merging this before codecov has a fix, then I suggest we document that we need to come back to adding code coverage for python 3.8 in an issue.

[bdice]

If we plan on merging this before codecov has a fix, then I suggest we document that we need to come back to adding code coverage for python 3.8 in an issue.

Agreed - I was documenting it for that reason. #259 has been created.

[codecov]

Codecov Report

Merging #258 into master will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #258      +/-   ##
==========================================
- Coverage   65.08%   65.03%   -0.06%     
==========================================
  Files          39       39              
  Lines        5588     5588              
==========================================
- Hits         3637     3634       -3     
- Misses       1951     1954       +3

Impacted Files	Coverage Δ
signac/contrib/project.py	90.42% <0%> (-0.45%)	⬇️

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