Deprecate the support of python 3.6

[fangyixiao18]

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Motivation

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Modification

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BC-breaking (Optional)

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Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
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[codecov]

Codecov Report

Base: 81.84% // Head: 80.49% // Decreases project coverage by -1.35% ⚠️

Coverage data is based on head (916eff5) compared to base (a1f1dad).
Patch coverage: 74.04% of modified lines in pull request are covered.

❗ Current head 916eff5 differs from pull request most recent head 9731b93. Consider uploading reports for the commit 9731b93 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x     #657      +/-   ##
===========================================
- Coverage    81.84%   80.49%   -1.36%     
===========================================
  Files          120      141      +21     
  Lines         4176     5056     +880     
  Branches       586      671      +85     
===========================================
+ Hits          3418     4070     +652     
- Misses         645      851     +206     
- Partials       113      135      +22     

Flag	Coverage Δ
unittests	80.49% <74.04%> (-1.36%)	⬇️

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

Impacted Files	Coverage Δ
mmselfsup/datasets/transforms/__init__.py	100.00% <ø> (ø)
mmselfsup/datasets/transforms/wrappers.py	100.00% <ø> (ø)
mmselfsup/engine/hooks/deepcluster_hook.py	38.88% <0.00%> (-0.55%)	⬇️
mmselfsup/engine/optimizers/lars.py	16.12% <ø> (ø)
mmselfsup/models/algorithms/odc.py	31.70% <ø> (ø)
mmselfsup/models/backbones/mae_vit.py	100.00% <ø> (ø)
mmselfsup/models/backbones/simmim_swin.py	86.95% <ø> (ø)
mmselfsup/models/heads/latent_heads.py	100.00% <ø> (ø)
mmselfsup/models/losses/swav_loss.py	83.01% <ø> (ø)
mmselfsup/models/necks/densecl_neck.py	85.71% <ø> (ø)
... and 83 more

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