Adds a `timeout` parameter to XRootDSource, which can be used as following:
python
import uproot
f = uproot.open("root://wherever//whatever.root", xrootdsource={"timeout": 1})
Also, *Source.defaults have been turned into dicts, rather than functions, and dict-based parameters like `xrootdsource` in the above example override defaults, rather than replacing them completely. That way, you don't need to specify the `chunkbytes` and `limitbytes` if you don't want to.