Add option to silence warnings related to deprecation of Python versions

tests/functional/test_warning.py

@@ -28,3 +28,39 @@ def test_deprecation_warnings_can_be_silenced(script, warnings_demo):
     script.environ['PYTHONWARNINGS'] = 'ignore'
     result = script.run('python', warnings_demo)
     assert result.stderr == ''
+
+
+deprecation_message = "DEPRECATION: Python 2.7 will reach the" \
+                      " end of its life on January 1st, 2020." \
+                      " Please upgrade your Python as Python 2.7" \
+                      " won't be maintained after that date. " \
+                      "A future version of pip will drop support" \
+                      " for Python 2.7. More details about Python 2" \
+                      " support in pip, can be found at" \
+                      " https://pip.pypa.io/en/latest/development/release-process/#python-2-support"  # noqa
+
+
+@pytest.mark.skipif(
+    "sys.version_info[:2] in [(2, 7)] "
+    "and platform.python_implementation() == 'CPython'")
+def test_version_warning_is_not_shown_if_python_version_is_not_27(script):
+    result = script.pip("debug", allow_stderr_warning=True)
+    assert deprecation_message not in result.stderr, str(result)
+
+
+@pytest.mark.skipif(
+    "not (sys.version_info[:2] in [(2, 7)] "
+    "and platform.python_implementation() == 'CPython')")
+def test_version_warning_is_shown_if_python_version_is_27(script):
+    result = script.pip("debug", allow_stderr_warning=True)
+    assert deprecation_message in result.stderr, str(result)
+
+
+@pytest.mark.skipif(
+    "not (sys.version_info[:2] in [(2, 7)] "
+    "and platform.python_implementation() == 'CPython')")
+def test_version_warning_is_not_shown_when_flag_is_passed(script):
+    result = script.pip("debug", "--no-python-version-warning",
+                        allow_stderr_warning=True)
+    assert deprecation_message not in result.stderr, str(result)
+    assert "--no-python-version-warning" not in result.stderr