pytest-runner shouldn't be required for setup

[yurivict]

It de-facto builds fine without it.

Found the problem while creating the FreeBSD port.

[amirkdv]

I second this! This is unnecessarily complicating our build process (we depend on datrie through snakemake). pytest-runner is clearly only needed for development and belongs in extras_require rather than setup_requires.

Current behavior

$ cat requirements.txt
snakemake
$ pip install -r requirements.txt
# fails, because pytest-runner is not installed for datrie

$ pip install pytest-runner
$ pip install -r requirements.txt
# succeeds

Desired behavior

$ cat requirements.txt
snakemake
$ pip install -r requirements.txt
# should succeed

[amirkdv]

^ Full disclosure: strange; I encountered the above problem yesterday and now cannot even reproduce it (building in isolated environments using docker, so there is no "memory" from previous pip invocations).