Move the package to the ‘src’ dir.

This allows testing against the installed package instead of the version controlled source.

Other projects which follow this are for example:
 - https://github.com/pyca/cryptography (See https://github.com/pyca/cryptography/commit/c62a78c015cf7aeb0c05bce82ef14cd86fe0b0fc)
 - https://github.com/getsentry/sentry