Support for EU/AU regions

[eliemada]

Currently the API base URL is hardcoded to a single region:

API_BASE_URL = "https://eu.edstem.org/api/"

This means users on other regions (US, AU) need to manually edit the source.

Proposal:

Add a region parameter to EdAPI.__init__:

class EdAPI:
    def __init__(self, region: str = "us"):
        subdomain = "" if region == "us" else f"{region}."
        self.api_base_url = f"https://{subdomain}edstem.org/api/"
        self.static_file_base_url = f"https://static.{subdomain}edusercontent.com/files/"
        # ...

Then update all urljoin(API_BASE_URL, ...) calls to use self.api_base_url.

Happy to submit a PR if you're open to this.