test_peg_generator leaks references.

Example with s390x RHEL8 Refleaks 3.x:
https://buildbot.python.org/all/#builders/536/builds/67

test_peg_generator leaked [24246, 24242, 24246] references, sum=72734
test_peg_generator leaked [10928, 10926, 10928] memory blocks, sum=32782


The following test is enough to trigger a leak:

$ ./python -m test -R 3:3 test_peg_generator -m test.test_peg_generator.test_c_parser.TestCParser.test_error_in_rules

0:00:00 load avg: 0.94 Run tests sequentially
0:00:00 load avg: 0.94 [1/1] test_peg_generator
beginning 6 repetitions
123456
......
test_peg_generator leaked [55, 55, 55] references, sum=165
test_peg_generator leaked [19, 19, 19] memory blocks, sum=57
test_peg_generator failed

== Tests result: FAILURE ==

1 test failed:
    test_peg_generator

Total duration: 10.8 sec
Tests result: FAILURE