Use native string type for envvars passed to Popen.
Review Request #10548 — Created May 8, 2019 and submitted — Latest diff uploaded
On Linux and Mac, Python is pretty flexible about whether the
environment dictionary contains bytestrings or unicode, but Windows is
stricter. We were initially forcing everything to bebytes, but Python
3 on Windows wantsstr, so this change switches to that instead.
Ran unit tests.