Cloud-files

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* docs: describe GCS composite and S3 multi-part uploads
* docs: update highlights, show cloudfiles
* install: update supported python versions
* install: adjust python requires to be 3.7+
* feat: make s3 multi-part threshold configurable
* feat: add support for CLOUD\_VOLUME\_DIR and CLOUD\_FILES\_DIR env vars (83)
* feat: multi-part s3 upload support (85)
* feat: support for GCS composite uploads (86)
* feat: add and remove persistent aliases (84)
* feat: http basic auth support (82)

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* feat: add clear\_memory function

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* feat: add .protocol to CloudFile object
* chore: update ChangeLog

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* feat: simplify interface to ResumableTransfer.init
* docs: show how to use resumable xfer
* feat: resumable transfers (81)
* feat: multipart validation (79)
* fix(mem): clear the real instance when resetting the connection pool
* fix: CloudFile.put now works
* build: update python versions tested

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* fix(s3): skip empty filenames in list\_files

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* feat(CloudFile): add len method and cache size option