Pass --python-version to pip install (along with --only-binary=:all:)

[pdecat]

This allows to package dependencies like rtoml with compiled binaries when local python version differs from Lambda runtime python version.

Without this change with python 3.9 locally targetting python 3.8 Lambda runtime:

# unzip -t .terraform/modules/my.lambda-notify/builds/b002507ad36e4a2c5527e154c21d5e88215c4647ca39461c89a1031eddabc806.zip | grep rtoml.*.so
    testing: rtoml/_rtoml.cpython-39-x86_64-linux-gnu.so   OK

With this change:

unzip -t .terraform/modules/my.lambda-notify/builds/95f1ec0c95f09c8e5832de7d5ef605cf20be46d8015bfa704782e4f33f860d10.zip | grep rtoml.*.so
    testing: rtoml/_rtoml.cpython-38-x86_64-linux-gnu.so   OK

Related: #23