setup: disable test command

setup.py

@@ -3,33 +3,41 @@ import codecs
 import sys
 from os import environ, path
 from sys import argv, path as sys_path
+from textwrap import dedent
 
 from setuptools import find_packages, setup
 
 import versioneer
 
 
+def format_msg(text, *args, **kwargs):
+    return dedent(text).strip(" \n").format(*args, **kwargs)
+
+
 CURRENT_PYTHON = sys.version_info[:2]
 REQUIRED_PYTHON = (3, 6)
 
 # This check and everything above must remain compatible with older Python versions
 if CURRENT_PYTHON < REQUIRED_PYTHON:
-    # noinspection PyStringFormat
-    sys.exit(
-        """
-========================================================
-               Unsupported Python version
-========================================================
-This version of Streamlink requires at least Python {}.{},
-but you're trying to install it on Python {}.{}.
+    sys.exit(format_msg("""
+        ========================================================
+                       Unsupported Python version
+        ========================================================
+        This version of Streamlink requires at least Python {}.{},
+        but you're trying to install it on Python {}.{}.
 
-This may be because you are using a version of pip that
-doesn't understand the python_requires classifier.
-Make sure you have pip >= 9.0 and setuptools >= 24.2
-        """
-        .strip(" \n")
-        .format(*(REQUIRED_PYTHON + CURRENT_PYTHON))
-    )
+        This may be because you are using a version of pip that
+        doesn't understand the python_requires classifier.
+        Make sure you have pip >= 9.0 and setuptools >= 24.2
+    """, *(REQUIRED_PYTHON + CURRENT_PYTHON)))
+
+# Explicitly disable running tests via setuptools
+if "test" in sys.argv:
+    sys.exit(format_msg("""
+        Running `python setup.py test` has been deprecated since setuptools 41.5.0.
+        Streamlink requires pytest for collecting and running tests, via one of these commands:
+        `pytest` or `python -m pytest` (see the pytest docs for more infos about this)
+    """))
 
 
 data_files = []