Return a dict of environment proxies. From requests_.

def get_environ_proxies():
    """Return a dict of environment proxies. From requests_."""

    proxy_keys = [
        'all',
        'http',
        'https',
        'ftp',
        'socks',
        'ws',
        'wss',
        'no'
    ]

    def get_proxy(k):
        return os.environ.get(k) or os.environ.get(k.upper())

    proxies = [(key, get_proxy(key + '_proxy')) for key in proxy_keys]
    return dict([(key, val) for (key, val) in proxies if val])