Implementing __rpow__

See merge request inducer/pycuda!58

Update compyte for strides on zero-sized arrays

See merge request !57

Implementing stack() and concatenate()

See merge request !56

docs: fix a few simple typos

There are small typos in:
- aksetup_helper.py
- examples/from-wiki/mandelbrot_interactive.py
- pycuda/cuda/pycuda-helpers.hpp
- test/test_gpuarray.py

Fixes:
- Should read `subtraction` rather than `substraction`.
- Should read `support` rather than `supprt`.
- Should read `right` rather than `rigth`.
- Should read `preferred` rather than `prefered`.

Closes #294

Update dev attr

See merge request inducer/pycuda!55

Fix a py2-ism in the tutorial

Update compyte

See merge request inducer/pycuda!54

Run flake8 in Github CI on target Py version