scripts: requirements.txt: fix PyYAML for python 3.7

PyYAML 3.13 includes a fix necessary to install on Python 3.7:

    3.13 (2018-07-05)

    Rebuild wheels using latest Cython for Python 3.7 support.

Support it by accepting any PyYAML at least as recent as the current
version in requirements.txt, currently 3.12.

Signed-off-by: Marti Bolivar <marti@foundries.io>