Remove pytest-runner dependency from setup.py

[ai-athena-oracle]

In Python library of Seldon, there is a line in setup.py which says setup_requires=["pytest-runner"]. This is only needed if one wants to run tests this way python setup.py pytest but it bring another main dependency to the package. You already have pytest dependency in tests_require , which is a cleaner practice to add test dependencies. This allow
to run tests this way pytest tests, which has the same effect as the other approach. Bringing an extra dependency (especially, which is only used for tests) to the main package introduces an additional issue for big companies, which require taking each dependency through rigorous approval process.

Another argument for removing pytest-runner is that this package has a deprecation notice:

pytest-runner depends on deprecated features of setuptools and relies on features that break security mechanisms in pip. For example 'setup_requires' and 'tests_require' bypass pip --require-hashes. See also https://github.com/pypa/setuptools/issues/1684.

[ai-athena-oracle]

Duplicate to #2302