Fail fast when an invalid API access key is provided

To replace existing behaviour where we fail silently and try the next available method of providing an API access key, ultimately falling back to anonymous access.

Also, updated the README to clean it up and make it clearer.

Fix broken logging functionality

The library uses print statements by default for logging, but allows users to change that behavior by providing their own logging function when instantiating ecmwfapi.ECMWFDataServer and ecmwfapi.ECMWFService. That functionality was broken in the sense that it didn't apply to all the produced logging messages, and some of them still used print, instead of the user provided logging function, no matter what. The issue should now be solved.

Python 3.9 support

v1.6.1

Python 3.9 support

Anonymous access

Enable anonymous access.

Integrate some user pull requests

Integrate some user pull requests to remove whitespace, fix spelling, and drop the use of simplejson.

Further improvements on PyPi integration

v1.5.3

Version 1.5.3

Enable PyPi display of README

v1.5.2

Version 1.5.2 - allow display README.md on PyPi