mdv: migrate to python@3.9

[fxcoudert]

As part of the Python 3.9 migration (#62201).

This formula is independent from the all other Python formulas (if I didn't screw up my script or my logic)

Do not merge before the next Brew tag ships, expected on Monday 2020-10-12

[fxcoudert]

    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/private/tmp/pip-req-build-8im_0e0r/setup.py", line 234, in <module>
        setup(
      File "/usr/local/Cellar/python@3.9/3.9.0/Frameworks/Python.framework/Versions/3.9/lib/python3.9/distutils/core.py", line 148, in setup
        dist.run_commands()
      File "/usr/local/Cellar/python@3.9/3.9.0/Frameworks/Python.framework/Versions/3.9/lib/python3.9/distutils/dist.py", line 966, in run_commands
        self.run_command(cmd)
      File "/usr/local/Cellar/python@3.9/3.9.0/Frameworks/Python.framework/Versions/3.9/lib/python3.9/distutils/dist.py", line 985, in run_command
        cmd_obj.run()
      File "/usr/local/Cellar/mdv/1.7.4_1/libexec/lib/python3.9/site-packages/setuptools/command/install.py", line 61, in run
        return orig.install.run(self)
      File "/usr/local/Cellar/python@3.9/3.9.0/Frameworks/Python.framework/Versions/3.9/lib/python3.9/distutils/command/install.py", line 546, in run
        self.run_command('build')
      File "/usr/local/Cellar/python@3.9/3.9.0/Frameworks/Python.framework/Versions/3.9/lib/python3.9/distutils/cmd.py", line 313, in run_command
        self.distribution.run_command(command)
      File "/usr/local/Cellar/python@3.9/3.9.0/Frameworks/Python.framework/Versions/3.9/lib/python3.9/distutils/dist.py", line 985, in run_command
        cmd_obj.run()
      File "/usr/local/Cellar/python@3.9/3.9.0/Frameworks/Python.framework/Versions/3.9/lib/python3.9/distutils/command/build.py", line 135, in run
        self.run_command(cmd_name)
      File "/usr/local/Cellar/python@3.9/3.9.0/Frameworks/Python.framework/Versions/3.9/lib/python3.9/distutils/cmd.py", line 313, in run_command
        self.distribution.run_command(command)
      File "/usr/local/Cellar/python@3.9/3.9.0/Frameworks/Python.framework/Versions/3.9/lib/python3.9/distutils/dist.py", line 985, in run_command
        cmd_obj.run()
      File "/private/tmp/pip-req-build-8im_0e0r/setup.py", line 184, in run
        out = template % self._get_context(src, outfile)
      File "/private/tmp/pip-req-build-8im_0e0r/setup.py", line 116, in _get_context
        c['body'] = self.md.convert(src)
      File "build/lib/markdown/__init__.py", line 375, in convert
        newRoot = treeprocessor.run(root)
      File "build/lib/markdown/treeprocessors.py", line 361, in run
        brs = root.getiterator('br')
    AttributeError: 'xml.etree.ElementTree.Element' object has no attribute 'getiterator'
    Running setup.py install for Markdown: finished with status 'error'
ERROR: Command errored out with exit status 1: /usr/local/Cellar/mdv/1.7.4_1/libexec/bin/python -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/private/tmp/pip-req-build-8im_0e0r/setup.py'"'"'; __file__='"'"'/private/tmp/pip-req-build-8im_0e0r/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' install --record /private/tmp/pip-record-q346dbmz/install-record.txt --single-version-externally-managed --compile --install-headers /usr/local/Cellar/mdv/1.7.4_1/libexec/include/site/python3.9/Markdown Check the logs for full command output.

[fxcoudert]

Reported upstream: axiros/terminal_markdown_viewer#86

[fxcoudert]

No news on the upstream report. Closing, will wait for their fix to migrate.