Only require pytest-runner if running under pytest

In some environments, `setup_requires=['pytest-runner']` can result in
pytest-runner being installed as a result of installing flake8 even
though it wouldn't otherwise be required.  The pytest-runner
documentation recommends a conditional `setup_requires` to avoid this,
so use that:

  https://pypi.org/project/pytest-runner/#conditional-requirement
Status Job ID Name Coverage
  Test
passed #129865351
docs

00:01:35

passed #129865352
dogfood

00:01:11

passed #129865350
linters

00:01:58

passed #129865345
python2

00:01:17

passed #129865346
python34

00:01:18

passed #129865347
python35

00:01:14

passed #129865348
python36

00:01:19

passed #129865349
python37

00:01:18

 
  Build
passed #129865353
build

00:01:13