Run flake8 in Github CI on target Py version

Implement pyopencl.array.stack

OpenCL 3 in wheels

Drop dependency on decorator module

Test, fix allocations >= 2**32 bytes in mempool

Fix, test passing precompiled headers to Program.compile

Intel CL Github CI job

Clarify caching semantics (closes gh-443)

Co-authored-by: Matthias Diener <mdiener@illinois.edu>

move directory creation inside the wheel context

copy scripts to lib directory