py-astunparse: @1.6.3: test failures for new Python 3.9 subport

[mascguy (Christopher Nielsen)]

While new subport py39-astunparse builds successfully, numerous tests fail.

Of note, tests succeed for subports < 3.9, suggesting that 3.9 compatibility isn't quite there yet. That was also supported by my research into Python 3.9 compatibility for ports such as py-astunparse, among other key dependencies for py-tensorflow, two weeks ago.

If the Python 3.9 subport is intended to be a placeholder, with the caveat that it may not be fully functional yet, that's fine. (With the caveat that a port note, succinctly stating that, probably should be added.)

However, if the hope was to provide a 100% functional subport - particularly to support tensorflow - that may not be possible quite yet. (Important note: Our port for py-astunparse is already using the latest released version - 1.6.3 - so we can't upgrade it at the present time.)

Logs attached for two test runs:

• Tests for py38-astunparse, which succeed.
• Tests for py39-astunparse, which fail.