setup: Remove deprecated 'setup.py test' command.

This was added in the first version of setup.py for the project. Since then,
setuptools has focused more on becoming a build tool (see eg.
pypa/setuptools#1684), and the project has developed a more refined testing
infrastructure which doesn't explicitly use this functionality.

If remaining, this code would require a noqa comment for compliance with the
incoming `N` set of linting rules, which is now no longer necessary.

setup.py

@@ -1,29 +1,11 @@
 import codecs
 import os
-import sys
 
 from setuptools import find_packages, setup
-from setuptools.command.test import test as TestCommand
 
 from zulipterminal.version import ZT_VERSION
 
 
-class PyTest(TestCommand):
- user_options = [("pytest-args=", "a", "Arguments to pass to pytest")]
-
- def initialize_options(self):
- TestCommand.initialize_options(self)
- self.pytest_args = ""
-
- def run_tests(self):
- import shlex
-
- import pytest
-
- errno = pytest.main(shlex.split(self.pytest_args))
- sys.exit(errno)
-
-
 def long_description():
  if not (os.path.isfile("README.md") and os.access("README.md", os.R_OK)):
  return ""
@@ -97,8 +79,6 @@ def long_description():
  keywords="",
  packages=find_packages(exclude=["tests", "tests.*"]),
  zip_safe=True,
- cmdclass={"test": PyTest},
- test_suite="test",
  entry_points={
  "console_scripts": [
  "zulip-term = zulipterminal.cli.run:main",