Ignore .pyc files

[twm]

It doesn't seem like incremental should mangle bytecode files:

❯ .tox/twine/bin/python -m incremental.update --rc treq
Updating codebase to 20.9.0rc1
Updating /home/twm/dev/treq/src/treq/__pycache__/api.cpython-37.pyc
Updating /home/twm/dev/treq/src/treq/__pycache__/api.cpython-36.pyc

Observed with incremental 17.5.0

[wiml]

+1. In fact it should probably have several exclusions, off the top of my head these seem reasonable:

• don't mess with .pyc files
• possibly don't do the Version(..., NEXT, ...) substitution if it isn't in a .py file?
• don't recurse into __pycache__ at all
• don't touch *~ or other backup-like files
• possibly don't even recurse into subdirectories that don't have an __init__.py ?

Perhaps we should implement the most-obvious of these and then allow exclusion patterns to be added in a setup.cfg section if it turns out some projects have more complicated needs

[twm]

The ability to exclude specific files would be useful, for example so that the Twisted compatibility policy can include examples of the NEXT substitution syntaxes.