Python 3.9 support on Linux

[PumpkinCheshire]

Concisely describe the proposed feature
Release the python 3.9 support for Linux. I noticed there is no 0.7.20-cp39-manylinux1 wheels in pypi, therefore i received

ERROR: Could not find a version that satisfies the requirement taichi (from versions: none)
ERROR: No matching distribution found for taichi 

Describe the solution you'd like (if any)
Release the latest cp39-manylinux wheels on pypi

Additional comments
Thanks

[Leonz5288]

We just released version 0.7.21 which supports python 3.9! Feel free to upgrade and try it out.

[PumpkinCheshire]

Sorry to interrupt. It seems the taichi-0.7.25-cp39-cp39-manylinux1_x86_64.whl is missing.. again. Is this intentional or an unexpected error.
If this is intentional, and the update around linux python 3.9 will reappear in the next version, I will not ask this question again.

[k-ye]

Hey @PumpkinCheshire

It's a limitation in our Linux release pipeline, where it's quite hard to restart a failed release. Hopefully the next release
will cover all the Python versions. Sorry about the inconvenience