This test passed locally because I had a python environment with the
`python` command available, but I should have used the `%PYTHON` lit
command substitution instead. Fixes buildbot failures from #163620.
Adds initial support for Python bindings to the OpenACC dialect.
* The bindings do not provide any niceties yet, just the barebones
exposure of the dialect to Python. Construction of OpenACC ops is
therefore verbose and somewhat inconvenient, as evidenced by the test.
* The test only constructs one module, but I attempted to use enough
operations to be meaningful. It does not test all the ops exposed, but
does contain a realistic example of a memcpy idiom.
