Specify pre-release branch for backports.tempfile
Specify pre-release branch for backports.tempfile

From the pip docs:
https://pip.pypa.io/en/stable/reference/pip_install/#pre-release-versions

>Starting with v1.4, pip will only install stable versions as specified
>by PEP426 by default. If a version cannot be parsed as a compliant
>PEP426 version then it is assumed to be a pre-release.
>
>If a Requirement specifier includes a pre-release or development version
>(e.g. >=0.0.dev0) then pip will allow pre-release and development
>versions for that requirement.

This change assures that pip won't fail while installing
backports.tempfile, which is at version 1.0rc1

While the behavior of different pip versions differs with regard to
pre-release versions, this gist shows an example of a failure:

https://gist.github.com/mckayward/7e83668a7bf74a3712c99e4dc6a71430

houqp/floyd-cli,houqp/floyd-cli,mckayward/floyd-cli,mckayward/floyd-cli