[Fix] Fix mdformat version to support python3.6

[jbwang1997]

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Motivation

Fix mdformat version to support python3.6.

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #8195 (152477a) into dev (2b05497) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #8195   +/-   ##
=======================================
  Coverage   64.17%   64.17%           
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  Files         361      361           
  Lines       29526    29526           
  Branches     5019     5019           
=======================================
  Hits        18948    18948           
  Misses       9575     9575           
  Partials     1003     1003           

Flag	Coverage Δ
unittests	64.15% <ø> (ø)

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