textproc/ibus-typing-booster: Adjust *_DEPENDS for Python 3.8+

• Since Python 3.8.7 (and also 3.9.2), .so files get an extension, e.g. ".cpython-38.so" if built with Python 3.8.
  
  Thus check for the presence of the Python packages instead of the actual .so files.

PR: 253815
Approved by: thierry (maintainer, via e-mail)