Switch to New Python 3.8 Enviroment

[tajmone]

The package is currently using the default Python environment (Python 3.3.6), but we should switch to the latest Python 3.8 environment used by Sublime Text 4, since this is a ST4 package.

• Create .python-version file in root containing 3.8 string.
• Check that all current macros work fine:
  • macros/Transform Word - Bold.sublime-macro
  • macros/Transform Word - Italic.sublime-macro

The switch shouldn't break anything, since we don't have any Python modules in the package. But to be on the safe — since I don't know the ST API and whether the switch might affect the syntax of our current macros — we should check that macros don't break due to the Python version switch.