This commit removes the old parser, the deprecated parser module, the old parser compatibility flags and environment variables and all associated support code and documentation.
Previously every test was building an extension module and
loading it into sys.modules. The tearDown function was thus
not able to clean up correctly, resulting in memory leaks.
With this commit, every test function now builds the extension
module and runs the actual test code in a new process
(using assert_python_ok), so that sys.modules stays intact
and no memory gets leaked.