Skip tests with pygments version mismatch.

If pygments is installed and the version doesn't match the expected version.
then any relevant tests will fail. To avoid failing tests due to different
output by pygments, those tests will be skipped. The pygments tox env
sets the `PYGMENTS_VERSION environment variable, so that env will always
run those tests against the expected version.

tests/test_syntax/extensions/test_code_hilite.py

@@ -21,17 +21,27 @@
 
 from markdown.test_tools import TestCase
 from markdown.extensions.codehilite import CodeHiliteExtension, CodeHilite
+import os
 
 try:
     import pygments  # noqa
     has_pygments = True
 except ImportError:
     has_pygments = False
 
+# The version required by the tests is the version specified and installed in the 'pygments' tox env.
+# In any environment where the PYGMENTS_VERSION environment variabe is either not defined or doesn't
+# match the version of Pygments installed, all tests which rely in pygments will be skipped.
+required_pygments_version = os.environ.get('PYGMENTS_VERSION', '')
+
 
 class TestCodeHiliteClass(TestCase):
     """ Test the markdown.extensions.codehilite.CodeHilite class. """
 
+    def setUp(self):
+        if has_pygments and pygments.__version__ != required_pygments_version:
+            self.skipTest(f'Pygments=={required_pygments_version} is required')
+
     maxDiff = None
 
     def assertOutputEquals(self, source, expected, **options):
@@ -340,6 +350,10 @@ def test_codehilite_startinline(self):
 class TestCodeHiliteExtension(TestCase):
     """ Test codehilite extension. """
 
+    def setUp(self):
+        if has_pygments and pygments.__version__ != required_pygments_version:
+            self.skipTest(f'Pygments=={required_pygments_version} is required')
+
     maxDiff = None
 
     def testBasicCodeHilite(self):