Appstoreconnect

Bugfixes:
- Relax cryptography dependencies (conformist-mw)
- Do not assume presence of content-type header (jaysoffian)
- Fix a possible RecursionError

Features:
- Add a timeout parameter (in seconds) for requests
- Add proxy support

Bugfixes:
- Avoid a RecursionError when accessing an unknown attribute on some resources

Bugfixes:
- Relax required dependencies in setup.py
- Fix APIError exception handling
- Add relationships attribute to the Build resource

Features:
- New endpoint: `modify_user_account`
- Support getting more related resources, like `user.visibleApps()`

Bugfixes:
- Pin dependencies versions in setup.py

Features:
- Expose the HTTP status code when App Store Connect API response raises APIError (GClunies)

Bugfixes:
- Fix invite_user method (AricWu)