Pynytimes

- Automatically send correct version header
- Remove tests for Python 3.9
- Add correct classifiers for PyPy

- Fix Article Search (1)
- Use exponential backoff: This should help solve problems when you get the HTTP Error 429 (Too Many Requests), it automatically slows down requests.
- Update to version 0.3
- requests has been updated from version 2.22.0 to 2.23.0.

There are some changes, but there are no breaking changes. The only change that could potentially be breaking is the option "rate_limiting": False in article search. This options has been disabled, the library will now continuously make requests until it gets the status code 429, then it'll use the exponential backoff.

- Some bugfixes and updates that reflect the changes from the New York Times API.

Thanks to matnel for his contribution.

- Article Search now supports multiple sources.

This is the first "stable" version that I'm releasing on GitHub.