Remove unnecessary astpretty dependency

[ptheywood]

Astpretty is currently a pyflamegpu dependency in setup.py.in.

It is not actually used within pyfalemgpu's codegen module, only in a test.

It therefore is not a dependency of pyflamegpu, so should be removed from setup.py.in.

Usage within the test suite is also condition, guarded by an undefined variable DEBUG_OUT. This should be removed too, and can be added back in with the import if/when required.

Usage within the test suite is guarded by a file-scoped variable DEBUG_OUT, which is set to true.
We can make this an optional dependency installed in the test environment alongside pytest.

[Robadob]

This impacts #1131 too

[ptheywood]

This impacts #1131 too

It explains the comment in the conda recipe

astpretty # [linux, win64] Not clear why conda thinks this isn't used

I.e. conda is correct, its not used.