Add Python version notice

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 Terminal flashcards from Markdown.
 
 ## Installation
-Lightcards is only tested and maintained on Linux. Support for other Unix-like operating systems such as MacOS and the various BSDs is likely, although untested. Windows support may be possible with [windows-curses](https://pypi.org/project/windows-curses/) or the like, however much of the program makes the assumption that there is a Unix-like file hierarchy, which may break on Windows. Patches will not be accepted to improve Windows support, although would be for MacOS or a BSD.
+Lightcards is only tested and maintained on Linux. Support for other Unix-like operating systems such as MacOS and the various BSDs is likely, although untested. Windows support may be possible with [windows-curses](https://pypi.org/project/windows-curses/) or the like, however much of the program makes the assumption that there is a Unix-like file hierarchy, which may break on Windows. Nontrivial patches will not be accepted to improve Windows support, although may be for MacOS or BSD.
+
+Lightcards requires Python 3.9
 
 ### From PyPI
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