Release 2.0.6: Python 3.9 support and relicensing

This release sees the following changes:

• scikit-ued is being re-licensed from the MIT license to the GPLv3 license.
• The fast argument to skued.align and skued.ialign has been deprecated. Its value has no effect anymore.
• Official support for Python 3.9.
• Removed explicit requirement for the tifffile package.