plpython subtransaction test failure with Python 3.10

Originally reported as https://bugzilla.redhat.com/show_bug.cgi?id=1959080
during early integration testing of Python 3.10 done in Fedora.

The test failed because Python 3.10 reports the traceback a bit differently:

cat postgresql-13.2/src/pl/plpython/regression.diffs
diff -U3
postgresql-13.2/src/pl/plpython/expected/python3/plpython_subtransaction.out
/builddir/build/BUILD/postgresql-13.2/src/pl/plpython/results/python3/plpython_subtransaction.out
---
postgresql-13.2/src/pl/plpython/expected/python3/plpython_subtransaction.out
2021-05-11 17:05:24.116431883 +0200
+++
postgresql-13.2/src/pl/plpython/results/python3/plpython_subtransaction.out
2021-05-11 17:05:26.689459542 +0200
@@ -224,8 +224,8 @@
SELECT subtransaction_exit_subtransaction_in_with();
ERROR: ValueError: this subtransaction has already been exited
CONTEXT: Traceback (most recent call last):
- PL/Python function "subtransaction_exit_subtransaction_in_with", line 3,
in <module>
- s.__exit__(None, None, None)
+ PL/Python function "subtransaction_exit_subtransaction_in_with", line 2,
in <module>
+ with plpy.subtransaction() as s:
PL/Python function "subtransaction_exit_subtransaction_in_with"
-- Make sure we don't get a "current transaction is aborted" error
SELECT 1 as test;

The attached patch makes the traceback look the same in Python 3.10 and
also in earlier versions.

Regards,
Honza