Use PEP 508 python_version specs for test_requires

This allows us to specify requirements that depend upon the Python
version, so that the requirements can be expressed purely as
configuration and not as code.
10 jobs for pep-508-test-requires in 3 minutes and 14 seconds (queued for 12 seconds)
latest
Status Job ID Name Coverage
  Build
failed #559429
build:debian:jessie

00:03:06

passed #559430
build:debian:stretch

00:01:23

failed #559428
build:el7

00:00:49

passed #559427
build:tarball

00:00:36

 
  Test
skipped #559435
docs
skipped #559436
syntax
skipped #559431
test:python2.7
skipped #559432
test:python3.5
skipped #559433
test:python3.6
skipped #559434
test:python3.7
 
Name Stage Failure
failed
build:debian:jessie Build
/builds/leo-singer/gracedb-client /builds/leo-singer/gracedb-client
$ python setup.py sdist
/usr/lib/python2.7/distutils/dist.py:267: UserWarning: Unknown distribution option: 'long_description_content_type'
warnings.warn(msg)
error in ligo-gracedb setup command: 'tests_require' must be a string or list of strings containing valid project/version requirement specifiers
Running after script...
$ ls -l dist/
ls: cannot access dist/: No such file or directory
ERROR: Job failed: exit code 1
failed
build:el7 Build
Complete!
$ python setup.py sdist
/usr/lib64/python2.7/distutils/dist.py:267: UserWarning: Unknown distribution option: 'long_description_content_type'
warnings.warn(msg)
error in ligo-gracedb setup command: 'tests_require' must be a string or list of strings containing valid project/version requirement specifiers
Running after script...
$ ls -l dist/
ls: cannot access dist/: No such file or directory
ERROR: Job failed: exit code 1