Use a more modern approach to run tests (#1089)

Running tests through `python setup.py test` is deprecated:

> WARNING: Testing via this command is deprecated and will be removed
> in a future version. Users looking for a generic test entry point
> independent of test runner are encouraged to use tox.

I am not in favor of moving back to `tox`, we should simply run tests
using `python -m pytest` (or `make test`) and that's it.

A new extra was added, `dev`, to install development requirements:

    $ python -m pip install --upgrade --editable '.[dev]'

setup.py

@@ -4,39 +4,26 @@ import sys
 import codecs
 
 from setuptools import setup, find_packages
-from setuptools.command.test import test as TestCommand
 
 import httpie
 
-
-class PyTest(TestCommand):
-    """
-    Running `$ python setup.py test' simply installs minimal requirements
-    and runs the tests with no fancy stuff like parallel execution.
-
-    """
-
-    def finalize_options(self):
-        TestCommand.finalize_options(self)
-        self.test_args = [
-            '--doctest-modules',
-            '--verbose',
-            './httpie',
-            './tests',
-        ]
-        self.test_suite = True
-
-    def run_tests(self):
-        import pytest
-        sys.exit(pytest.main(self.test_args))
-
-
+# Note: keep requirements here to ease distributions packaging
 tests_require = [
-    'pytest-httpbin',
+    'docutils',
     'pytest',
+    'pytest-httpbin>=0.0.6',
+]
+dev_require = [
+    *tests_require,
+    'flake8',
+    'flake8-comprehensions',
+    'flake8-deprecated',
+    'flake8-mutable',
+    'flake8-tuple',
+    'pytest-cov',
+    'twine',
+    'wheel',
 ]
-
-
 install_requires = [
     'requests[socks]>=2.22.0',
     'Pygments>=2.5.2',
@@ -59,6 +46,7 @@ if 'bdist_wheel' not in sys.argv:
 
 # bdist_wheel
 extras_require = {
+    'dev': dev_require,
     'test': tests_require,
     # https://wheel.readthedocs.io/en/latest/#defining-conditional-dependencies
     ':sys_platform == "win32"': install_requires_win_only,
@@ -91,8 +79,6 @@ setup(
     python_requires='>=3.6',
     extras_require=extras_require,
     install_requires=install_requires,
-    tests_require=tests_require,
-    cmdclass={'test': PyTest},
     classifiers=[
         'Development Status :: 5 - Production/Stable',
         'Programming Language :: Python',