WIP: Patch open, os and os.path builtins

[pjbull]

Right now, libs that use open directly, fspath or just assume paths are strings and can be manipulated don't work with CloudPath objects. (Related issues: #315, #73, #128)

Totally WIP and not sure that we want to introduce and support this, but it may increase compatibility significantly to have the option at least for users to patch the builtin functions.

The biggest limitations are:

• Libs that do any string manipulation manually still will not work (e.g. path = dir + os.sep + file)
• Libs that pass local paths off to compiled code to do IO will still not work
• There are probably lots of other ways that other code can do funky things with paths, so even these patches may not make things work

Other potential todos for this PR:

• Patch the standard lib glob module as well
• Add integration test suite that shows the patches working with libs that did not work previously (pandas has all kinds of formats, rasterio may be interesting, xarray has come up, Pillow may be a common use case)
• Add env var for patching os functions
• override os.fspath to allow returning CloudPath
• Document these methods
• Add full test suite (can we leverage what CPython does for these functions?)
• Document that in Jupyter notebooks you have to do open = patch_open() since open exists as a local in the notebook namespace
• Remove testing script

Curious for your thoughts @jayqi !

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Experimental instructions

• pip install git+https://github.com/drivendataorg/cloudpathlib@patch-builtins
• Set the environment variable CLOUDPATHLIB_PATCH_OPEN=True before any import call to cloudpathlib
• Experiment with your code that uses open or third-party libraries that use open.
• Report the results in open(cloud_path) acts on cached local file instead of cloud file #128

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[pjbull]

So, the idea here is that even if code uses fspath to try to get the local path, we give back the CloudPath instead so it works downstream. This may break code that assumes fspath returns a string, which is what it is supposed to do.

(Also, note: this no longer caches on the call here so we only populate the cache on open).

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