windows: replicate the normalizing behavior of os.environ...
windows: replicate the normalizing behavior of os.environ
On Windows, `os.environ` normalizes environment variables to uppercase. Our
current bytes-based environ substitution object is a simple dict, so we add
the normalization behavior.
This fixes test-http-peer.t on Windows.
Differential Revision:
https://phab.mercurial-scm.org/D10998
