Custom record changes break python 3.9 #609

• asyncpg version: Built from github master this morning at midnight GMT
• PostgreSQL version: 13.0 beta but does not matter
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?: no but does not matter
• Python version: 3.9.0rc1+
• Platform: amd64
• Do you use pgbouncer?: no but does not matter
• Did you install asyncpg with pip?: built wheel from source
• If you built asyncpg locally, which version of Cython did you use?: 3.0a6
• Can the issue be reproduced under both asyncio and
  uvloop?: does not matter

I'm using asyncpg in a python 3.9 enviroment that I rebuild daily in CI. For reference, I use this docker image I rebuild from the 3.9 tree every day to produce the error.

Files for reference in case they are needed for reproducing:

• my Cython wheel for 3.9

The issue is that the latest commit regarding custom records seems to use C api methods that got moved in 3.9 (?). Since today, I get:

ImportError: Error relocating /home/idle/.local/lib/python3.9/site-packages/asyncpg/protocol/protocol.cpython-39-x86_64-linux-gnu.so: _PyObject_GC_IS_TRACKED: symbol not found

The function is referenced several times in the latest commit.

Relevant links:

• BPO link for the rename of the GC_IS_TRACKED and moving to public API
• 3.9 whatsnew section in the C API