Fix deprecation warnings

For `python-markdown`, this means bumping the minimum supported version to
3.0.0. For system dependencies, it means leaving python2 behind.

.travis.yml

@@ -10,6 +10,12 @@ matrix:
       python: 3.5
     - env: TOXENV=py36
       python: 3.6
+    - env: TOXENV=py37
+      python: 3.7
+    - env: TOXENV=py38
+      python: 3.8
+    - env: TOXENV=py39
+      python: 3.9
     - env: TOXENV=pypy
       python: pypy
     - env: TOXENV=pypy3

mdx_gh_links.py

@@ -32,7 +32,7 @@
 
 from markdown.extensions import Extension
 from markdown.inlinepatterns import Pattern
-from markdown.util import etree
+from xml.etree import ElementTree
 
 
 URL_BASE = 'https://github.com'
@@ -43,7 +43,7 @@
 
 
 def _build_link(label, title, href, classes):
-    el = etree.Element('a')
+    el = ElementTree.Element('a')
     el.text = label
     el.set('title', title)
     el.set('href', href)
@@ -121,11 +121,11 @@ def __init__(self, *args, **kwargs):
         }
         super(GithubLinks, self).__init__(*args, **kwargs)
 
-    def extendMarkdown(self, md, md_globals):
+    def extendMarkdown(self, md):
         md.ESCAPED_CHARS.append('@')
-        md.inlinePatterns['issue'] = IssuePattern(self.getConfigs(), md)
-        md.inlinePatterns['mention'] = MentionPattern(self.getConfigs(), md)
-        md.inlinePatterns['commit'] = CommitPattern(self.getConfigs(), md)
+        md.inlinePatterns.register(IssuePattern(self.getConfigs(), md), 'issue', 20)
+        md.inlinePatterns.register(MentionPattern(self.getConfigs(), md), 'mention', 20)
+        md.inlinePatterns.register(CommitPattern(self.getConfigs(), md), 'commit', 20)
 
 
 def makeExtension(*args, **kwargs):  # pragma: no cover

setup.py

@@ -44,18 +44,19 @@
     url='https://github.com/Python-Markdown/github-links/',
     license='BSD License',
     py_modules=['mdx_gh_links'],
-    install_requires=['markdown>=2.6.11'],
+    install_requires=['markdown>=3.0.0'],
     classifiers=[
         'Development Status :: 4 - Beta',
         'License :: OSI Approved :: BSD License',
         'Operating System :: OS Independent',
         'Programming Language :: Python',
-        'Programming Language :: Python :: 2',
-        'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3',
         'Programming Language :: Python :: 3.4',
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
+        'Programming Language :: Python :: 3.8',
+        'Programming Language :: Python :: 3.9',
         'Programming Language :: Python :: Implementation :: CPython',
         'Programming Language :: Python :: Implementation :: PyPy',
         'Topic :: Documentation',