Urllib verus requests dependency

[scottyhq]

it seems that both urllib (standard library) and requests (external dependency) are used in a few places. If possible, might be good to drop the extra requests dependency or stick with using it everywhere. cc @tsutterley

https://github.com/ICESat2-SlideRule/sliderule/blob/7e16d3e90efc932fc96219a2bba81daf040b44fa/clients/python/sliderule/earthdata.py#L34-L35

Discussed here #290 (comment)

[jpswinski]

The code using urllib is taken from the NSIDC and is a part of the code they provide for accessing the CMR services. The code using requests is what was written for SlideRule.

My preference would be to unify on requests because I don't think it is a cumbersome external dependency (not like some which complicate the dependency resolution or have issues on other platforms). It is also fairly standard at this point and is easier to use.