Remove astpretty as a dependency of pyflamegpu

[ptheywood]

astpretty is only used for prettyprint debugging in codegen testing - it is not a pyflamegpu dependency.

This commit also makes it an optional test_codegen.py dependency (but installs it in the test venv by default)

Closses #1164

[ptheywood]

pytest codegen tests still pass:

but if ran without astpretty installed, a message is output to the debug info (i.e. if someone doesn't build pyflamegpu, but installs pytest themselves and runs the test suite, so unlikely but no reason not to support it.)

python3 -m pytest ../tests/python/codegen/ -s
... 
collecting ... Install astpretty if ast tree printing required for test debugging
...
=========== 57 passed in 0.30s ==========

[Robadob]

Is there a reason to remove it from the venv, if it's used by the codegen tests?

[ptheywood]

this is adding it to the venv if pytest is also being installed.

Previously it would be implicitly installed as a dependency of pyflamegpu.

The changes to test_codegen.py itself making it optional within the test is not really neccesarry, just adds a bit more flexibility, it makes it so that just pyflamegpu and pytest are required, but if codegen debugging is wanted then astpretty might help.