tests: Patch `os.path.exists` and improve patching for `open`. You can specify additional modules for which to patch `open`. Also, it was incorrectly updating the opened file, even when it was opened for read only. Now it only updates the contents if the file was opened for write, and supports appending to the end. Last, it supports opening text files in binary mode.

syntax: glob
*.pyc
venv
tags
build/lib
build/messages/_cache
build/messages/_counter
dist
piecrust.egg-info
piecrust/__version__.py