[Feature] Add python3.9 in CI

[zhouzaida]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

Support python3.9

Modification

Add python3.9 in CI

BC-breaking (Optional)

No

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, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Merging #1291 (d7b75e3) into master (8aab4f2) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1291      +/-   ##
==========================================
+ Coverage   68.23%   68.27%   +0.04%     
==========================================
  Files         160      161       +1     
  Lines       10722    10742      +20     
  Branches     1969     1972       +3     
==========================================
+ Hits         7316     7334      +18     
- Misses       3022     3023       +1     
- Partials      384      385       +1     

Flag	Coverage Δ
unittests	68.27% <ø> (+0.04%)	⬆️

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

Impacted Files	Coverage Δ
mmcv/runner/builder.py	100.00% <0.00%> (ø)
mmcv/runner/__init__.py	100.00% <0.00%> (ø)
mmcv/runner/default_constructor.py	77.77% <0.00%> (ø)
mmcv/video/processing.py	77.41% <0.00%> (+0.37%)	⬆️
mmcv/runner/hooks/logger/pavi.py	71.42% <0.00%> (+0.41%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8aab4f2...d7b75e3. Read the comment docs.