[Fix] Fix CircleCI's OpenCV installation error on master branch.

[LeoXing1996]

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Motivation

As title. The modification refers to #1589.

Modification

Remove Python3.6 from setup.py and unit tests.

Who can help? @ them here!

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
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

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Base: 83.89% // Head: 83.89% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (f28faa1) compared to base (1e7e3d1).
Patch coverage: 86.15% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #1597    +/-   ##
========================================
  Coverage   83.89%   83.89%            
========================================
  Files         224      227     +3     
  Lines       12771    13027   +256     
  Branches     1994     2027    +33     
========================================
+ Hits        10714    10929   +215     
- Misses       1717     1742    +25     
- Partials      340      356    +16     

Flag	Coverage Δ
unittests	83.89% <86.15%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
mmedit/datasets/base_sr_dataset.py	76.47% <25.00%> (-23.53%)	⬇️
mmedit/core/registry.py	75.00% <75.00%> (ø)
mmedit/models/restorers/basic_restorer.py	95.74% <86.95%> (-2.89%)	⬇️
mmedit/core/evaluation/inception_utils.py	88.74% <88.74%> (ø)
mmedit/core/evaluation/inceptions.py	95.00% <95.00%> (ø)
mmedit/core/__init__.py	100.00% <100.00%> (ø)
mmedit/core/evaluation/__init__.py	100.00% <100.00%> (ø)
mmedit/core/evaluation/eval_hooks.py	64.28% <100.00%> (+2.74%)	⬆️
mmedit/models/common/generation_model_utils.py	95.19% <0.00%> (-4.81%)	⬇️

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