Add a consistent User-Agent header in Python SDK requests

[ixunio]

The node SDK contains a header User-Agent : mindee-node/${sdkVersion} node/${process.version} when making the request to Mindee API (api.mindee.net/v1).

We should add the same header so that Mindee API backend has the information.

Example:

User-Agent : mindee-python/v1.3.0 python/3.8.12

[ianardee]

fixed by #36