Create empty file if no bytes written to S3 file in 'w' mode

[chrisrink10]

This PR adds functionality to S3Boto3StorageFile that will create an empty file if no bytes are written to S3 after opening a file in w mode, similar to how FileSystemStorage and Python's native open work.

Let me know if I'm missing anything or if you have any questions. Thank you!

Fixes #435

[chrisrink10]

Looks like my remaining build failure is due to extra warnings being thrown on Python 3.8 for tests/test_s3boto3.py::S3Boto3Storagetests::test_deprecated_bucket. I'm not exactly clear how to proceed with fixing this issue. I don't think my changes caused this.

[jdufresne]

Why should this be optional? Shouldn't the backend always follow the upstream behavior of FileSystemStorage?

[chrisrink10]

@jdufresne Completely agree it should not be optional in an ideal world. I was concerned it would be a breaking change though, since that's not how it works right now. If people think it should be standard behavior I'm happy to make it that way (it's easier for me).

cc @jschneier

[blargoner]

@jschneier Any chance this can be merged? I love empty files.

[jschneier]

Please remove the setting.

…

On Monday, January 27, 2020, John Peloquin ***@***.***> wrote: @jschneier <https://github.com/jschneier> Any chance this can be merged? I love empty files. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#816?email_source=notifications&email_token=AAREDWFHV3R23GJQ4BWCZETQ74CHDA5CNFSM4KHJNEBKYY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOEKACWGA#issuecomment-578824984>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAREDWFSTGPWULORX7OJUXLQ74CHDANCNFSM4KHJNEBA> .

[chrisrink10]

@jschneier Thanks for the reply. I removed the setting.

Looks like the Python 3.8 build issue is resolved now.

[blargoner]

@jschneier Now that the setting is removed, any chance this can be merged? Thanks!