fix: Adding pillow as dependency

[jbusche]

Fixes issue #146

Testing:

git clone https://github.com/jbusche/mellea.git -b jbusche-add-pillow
cd mellea
python3.12 -m venv venv
source venv/bin/activate
pip install .
pip list |grep pill

Returns:

pillow 11.3.0

Now when I run, I can import mellea:

python
import mellea
m = mellea.start_session()
print(m.chat("How many letters in the English alphabet?").content)

Returns with:

There are 26 letters in the English alphabet.

[mergify]

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