Allow passing file contents as user_credentials, not just file paths

[roguh]

Implemented with tempfile.NamedTemporaryFile, open to new ideas. We can potentially get rid of tempfile and use the contents directly without relying on open().

https://security.openstack.org/guidelines/dg_using-temporary-files-securely.html

Hi, from PowerFlex.

[wallyqs]

Thanks for the PR, I was looking a how we could avoid the tempfile to support this usage. What do you think of something like this? e1c6924
This way usage would be:

await nats.connect(user_credentials=RawCredentials("<creds file contents as string>"))

[roguh]

That looks good. I would also prefer avoiding temp files, but wasn't sure how much of this code to change.

This also works well with type checkers and Python 3.7 I assume.