Defining a user-agent in GET requests

[elbeejay]

Aims to close #83 by adding a string to the header in our HTTP GET requests that identifies the python dataretrieval package and version making the request.

Checked functionality against what the R package does and this follows that convention. The "user-agent" information sent to the server is python-dataretrieval/<version> which is consistent with the approach in R.

Wrote some tests for this logic.

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@thodson-usgs can you confirm that my syntax for getting the current version is correct? We don't currently have a _version.py file so in the interest of having the code be more resilient there's currently try-except logic to substitute "version-unknown" when it is not available. But I am hoping that once we do the next release the code dataretrieval.__version__ will successfully return the version information as a string. Used similar logic to automatically set the versions for the hosted documentation that gets built each time a commit is made.

[thodson-usgs]

@elbeejay, something is wrong with my understanding. I swore that setuptools_scm writes _version.py and that was read by dataretrieval.__version__ but now I'm getting an AttributeError. I'll need to debug this further.

[elbeejay]

Updated the versioning references based on the documentation for setuptools_scm. This resulted in the retirement of support for Python 3.7, as the best practices for both obtaining the package version at runtime, as well as the dynamic loading of version information for the Sphinx documentation (per setuptools_scm documentation), require the use of the standard library functionality importlib.metadata, which was added in Python 3.8.

[thodson-usgs]

Please revise __init__.py

[thodson-usgs]

Nice, thanks @elbeejay!