Version 0.40.0: Better RTF Handling

v0.40.0

• [TeamMsgExtractor #338] Added new code to handle injection of text into the RTF body. For many cases, this will be much more effective as it relies on ensuring that it is in the main group and past the header before injection. It is not currently the first choice as it doesn't have proper respect for encapsulated HTML, however it will replace some of the old methods entirely. Solving this issue was done through the use of a few functions and the internal _rtf module. This module in it's entirety is considered to be implementation details, and I give no guarantee that it will remain in it's current state even across patch versions. As such, it is not recommended to use it outside of the module.
• Changed MessageBase.rtfEncapInjectableHeader and MessageBase.rtfPlainInjectableHeader from str to bytes. They always get encoded anyways, so I don't know why I had them returning as str.
• Updated minimum Python version to 3.8 as 3.6 has reached end of support and 3.7 will reach end of support within the year.
• Updated information in README.