update pillow to 7.2.0 from 6.2.2

[alexholdenmiller]

Patch description
update pillow version in requirements.py to 7.2.0 which is included with pytorch 1.4.0

Testing steps

conda create -n pillow python=3.7
source activate pillow
module purge && module activate cuda/10.1 cudnn/v7.6.5.32-cuda.10.1 NCCL/2.5.6-1-cuda.10.1
conda install pytorch=1.4.0 torchvision cudatoolkit=10.1 -c pytorch

python -m pip install --progress-bar off --upgrade pip setuptools
python -m pip install --progress-bar off coverage
python -m pip install --progress-bar off codecov
mkdir -p ~/ParlAI/test-results
pip install -v -r requirements.txt
python -c "import nltk; nltk.download('punkt')"

cd ~/ParlAI && python setup.py develop

python -m pytest -m unit

Logs
I had a number of weird transient test failures (mostly due to weird issues around installing things in a bad way) but there are no failures due to Pillow.

[alexholdenmiller]

(feel free to merge on accept)

[stephenroller]

<3