Fix nightly tool for python 3.6 (#55776)
Summary:
Given that the minimal required Python version for using PyTorch is 3.6, the development tools should also be able to handle it. `./tools/nightly.py` currently uses the parameters `capture_output` and `text` of `subprocess.run` that were only added for [Python 3.7](https://docs.python.org/3/library/subprocess.html#subprocess.run).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55776
Reviewed By: ngimel
Differential Revision: D27709124
Pulled By: ezyang
fbshipit-source-id: aeea15a891ba792f3cd5fa602f0d7b746007e30c
