Use native string type for envvars passed to Popen.
Review Request #10548 — Created May 8, 2019 and submitted
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 be
bytes, but Python
3 on Windows wants
str, so this change switches to that instead.
Ran unit tests.