py-pyaudio: add Python 3.9 subport

[radarhere]

Description

Closes: https://trac.macports.org/ticket/61937

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 11.1 20C69
Xcode 12.3 12C33

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?

• 'Error: Failed to test py-pyaudio: py-pyaudio has no tests turned on'
• Fairly certain there aren't any binary files.

[macportsbot]

Notifying maintainers:
@Jakker for port py-pyaudio.

[ryandesign]

When merging, please use the squash and merge button to reword the commit message to say "add" instead of "added".

[radarhere]

I've reworded the commit message.

[reneeotten]

thanks @radarhere ; next time please add the Closes: https://trac.macports.org/ticket/61937 in the body of the commit message so that the ticket will be closed automatically (I've now done that while merging the PR).