[Fix] Adjust the minimum supported python version to 3.7

[HAOCHENYE]

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Motivation

Adjust the minimum supported python version to 3.7

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

Base: 78.66% // Head: 78.02% // Decreases project coverage by -0.64% ⚠️

Coverage data is based on head (c2e80ca) compared to base (fe26c65).
Patch coverage: 46.87% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #863      +/-   ##
==========================================
- Coverage   78.66%   78.02%   -0.65%     
==========================================
  Files         128      130       +2     
  Lines        9348     9423      +75     
  Branches     1848     1886      +38     
==========================================
- Hits         7354     7352       -2     
- Misses       1679     1752      +73     
- Partials      315      319       +4     

Flag	Coverage Δ
unittests	78.02% <46.87%> (-0.65%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmengine/model/wrappers/__init__.py	66.66% <ø> (-3.34%)	⬇️
mmengine/optim/optimizer/zero_optimizer.py	52.17% <0.00%> (-4.97%)	⬇️
mmengine/registry/build_functions.py	80.20% <ø> (ø)
mmengine/registry/default_scope.py	100.00% <ø> (ø)
mmengine/runner/runner.py	84.91% <ø> (-0.24%)	⬇️
mmengine/structures/base_data_element.py	93.61% <ø> (-0.04%)	⬇️
mmengine/structures/label_data.py	100.00% <ø> (ø)
mmengine/visualization/vis_backend.py	83.79% <ø> (-0.70%)	⬇️
mmengine/visualization/visualizer.py	93.03% <ø> (-0.17%)	⬇️
mmengine/runner/checkpoint.py	52.07% <14.28%> (+8.11%)	⬆️
... and 51 more

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