19.10b0 starts to depend on gcc for pip installation under docker image python:3.7.5-slim-buster

[larrycai]

Describe the bug A clear and concise description of what the bug is.

this release needs gcc env to install library (regex3?) under popular docker image python:3.7.5-slim-buster, which makes complicated.

To Reproduce Steps to reproduce the behavior:

1. Simple reproduce in docker env, which is used in travis-ci, gitlab-ci nowadays

docker run python:3.7.5-slim-buster pip install black

2. Run Black on it with these arguments '....'
3. See error

   unable to execute 'gcc': No such file or directory
    error: command 'gcc' failed with exit status 1

Expected behavior A clear and concise description of what you expected to happen.

expected to install without extra gcc library, it works smoothly in previous 19.3b0 release

Environment (please complete the following information):

[hugovk]

Duplicate of #1112.