setup.py: require ruamel.yaml >= 0.15.41 < 0.15.52

      install_requires=[

          'setuptools',

          'psutil',

          'ruamel.yaml < 0.15.52',

          # According to ruamel.yaml's PyPI page, we are suppose to use

          # "<=0.15" in production until 0.15 becomes API stable.

          # However we need ruamel.yaml 0.15.41 or greater for Python 3.7.

          # We know that ruamel.yaml 0.15.52 breaks API in a way that

          # is incompatible with BuildStream.

          #

          # See issues #571 and #790.

          'ruamel.yaml >= 0.15.41, < 0.15.52',

          'pluginbase',

          'Click',

          'jinja2 >= 2.10',