fix: only send a restricted set of environment variables

[stympy]

Like our Ruby client, we need to prevent sending environment variables that aren't related to a web request.

[stympy]

@subzero10 can you arrange for someone who actually knows some Python to get the tests back to passing? :)

[subzero10]

@subzero10 can you arrange for someone who actually knows some Python to get the tests back to passing? :)

Yeap, I can do that.
Though, on first look it appears that some tests are failing because of the changes in this PR.

For example, this test is looking for
Content-Type, Content-Length and Host but they are not found. Content-Type is found in CGI_ALLOWLIST but it has a different format. I think the check could be improved:

def filter_env_vars(data):
    if type(data) != dict:
        return data

    filtered_data = {}
    for key, value in data.items():
        # Normalize key to uppercase with underscores
        normalized_key = key.upper().replace("-", "_")  
        if normalized_key.startswith('HTTP_') or normalized_key in CGI_ALLOWLIST:
            # Make sure to preserve the original key casing in the output
            filtered_data[key] = value  
    return filtered_data

[subzero10]

I think this is ready to merge :)