games/py-mnemosyne: Take Maintainer'ship, fix runtime error and add missing dependencies

Latest version of games/py-mnemosyne requires new dependencies. This commit adds the missing dependencies:

• audio/py-gtts
• security/py-gtts-token (required by audio/py-gtts)
• textproc/py-googletrans

The games/py-mnemosyne port needs to be limited to Python 3.7+ to keep the dependency chains intact. This is because textproc/py-googletrans depends on www/py-httpx which is only for Python 3.7+.

PR: 247595
Submitted by: kai
Reported by: gspurki@gmail.com
Approved by: tcberner (mentor)
MFH: 2020Q3
Differential Revision: https://reviews.freebsd.org/D25895