d2r raise Exception: AttributeError: 'str' object has no attribute 'supported'

[SergeyBridge]

Describe the bug
I want to convert .md into .rst format
I added 'd2r' extention, there was an Exception occurred
extensions = [
'sphinx.ext.todo',
'sphinx.ext.githubpages',
'sphinx.ext.napoleon',
'sphinx.ext.autodoc',
'jupyter_sphinx',
'sphinx.ext.viewcode',
'm2r',
]
After removing 'd2r' there was not any problems with config.py .

To Reproduce
Steps to reproduce the behavior:

<Paste your command-line here which cause the problem>
$ pip install m2r
Requirement already satisfied: m2r in /home/sergey/anaconda3/envs/ExParallel/lib/python3.8/site-packages (0.2.1)
Requirement already satisfied: docutils in /home/sergey/anaconda3/envs/ExParallel/lib/python3.8/site-packages (from m2r) (0.16)
Requirement already satisfied: mistune in /home/sergey/anaconda3/envs/ExParallel/lib/python3.8/site-packages (from m2r) (0.8.4)

$ sphinx-build /home/sergey/source/   /home/sergey/docs
Running Sphinx v3.5.3
loading translations [ru]... done

Exception occurred:
  File "/home/sergey/anaconda3/envs/ExParallel/lib/python3.8/site-packages/sphinx/registry.py", line 267, in add_source_parser
    for filetype in parser.supported:
AttributeError: 'str' object has no attribute 'supported'
The full traceback has been saved in /tmp/sphinx-err-uu_jfl9i.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!

**Expected behavior**
I expected my config.py running 

**Your project**
Link to your sphinx project, or attach zipped small project sample.
https://github.com/SergeyBridge/ExParallel/releases/tag/v0.01-alpha

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Environment info**
- OS: [Ubuntu 20.04 LTS]
- Python version: [Python 3.8.8]
- Sphinx version: [sphinx-build 3.5.3]
- Sphinx extensions:  [e.g. sphinx.ext.autodoc, recommonmark]
- Extra tools: [e.g. Browser, tex or something else]
extensions = [
    'sphinx.ext.todo',
    'sphinx.ext.githubpages',
    'sphinx.ext.napoleon',
    'sphinx.ext.autodoc',
    'jupyter_sphinx',
    'sphinx.ext.viewcode',
    'm2r',
]
**Additional context**
Add any other context about the problem here.

- [e.g. URL or Ticket]

[tk0miya]

It seems the latest release of m2r can not work with the latest Sphinx. Please check miyakogi/m2r#60.

Thanks,