Retry decorator for _call and _get_file

[oliverwm1]

Resolves #383 by retrying both _get_file and _call with a new retry_request decorator. Differently from #384, this PR leaves internals of _get_file intact.

[oliverwm1]

@martindurant let me know what you think of this as an alternative to #384

[martindurant]

On first look, this all seems reasonable. I have a couple of comments.
Unfortunately, the diff is a little inflated by the formatter changing some lines that were fine previously - perhaps a new version of black :|

[oliverwm1]

Unfortunately, the diff is a little inflated by the formatter changing some lines that were fine previously - perhaps a new version of black :|

yes, black's behavior when confronted with split lines that could fit into one line has changed. Anyway, I reverted the formatting changes.

[martindurant]

I reverted the formatting changes.

Still passes :)

[nbren12]

Is this ready to be merged? It would also be nice to issue a new PyPI release.

[oliverwm1]

I think it's good to go. @martindurant let me know if you see remaining issues—I'm happy to address.