Make pytest-runner a conditional requirement

[yegorich]

Having pytest-runner in setup_requires breaks mwclient build in Buildroot. Could you take similar approach as pytablewriter (thombashi/pytablewriter@63fe493#diff-2eeaed663bd0d25b7e608891384b7298)

[danmichaelo]

Thanks for the PR! Let me know if you need a new release, otherwise I'll wait with that until another change or two are in.

[yegorich]

@danmichaelo thanks for applying the fix. No need to hurry with the release. We've applied the patch locally and will remove it as soon as new release will appear.