Include STDOUT and PIPE in public API

[sethfischer]

Include STDOUT and PIPE in the public API as the documentation recommends their use.

This will mean linters do not produce warnings like the following:

'STDOUT' is not declared in __all__ 

Examples from the documentation recommending the used of STDOUT and PIPE:

Demo.ipynb:

from wurlitzer import pipes, sys_pipes, STDOUT, PIPE

README.md:

from wurlitzer import pipes, STDOUT

[minrk]

Thanks!

[minrk]

released 3.0.3 with this