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The problem is that the getproxies_environment function converts all environment variables to lower case before deciding whether to use the name. This means that whichever ends up last in os.environment will be used.
The attached patch does two things. It changes the behaviour to prefer the all lower case version if it exists, and adds a note to the ProxyHandler description noting that the environment variable used is not case sensitive, although the all lower case version will be preferred.
The behaviour change is debatable, but preferring the lower case name is likely to be less surprising in general.