use a python base image that has compilers, etc

Black recently added a dependency on regex, which doesn't have wheels for linux/CPy3.7. In https://bitbucket.org/mrabarnett/mrab-regex/issues/343/wheel-for-linux, the author suggests there are no plans to have one either for the current version or future versions. To insulate ourselves from this, we'll use the default Python Docker base image which contains gcc, etc.

Note that this doesn't mean that we can go to prod with packages that require compilers during install; we build the prod Docker image with python-alpine, which doesn't contain any compilers, so any packages missing wheels we'd need to deal with separately there.