build: force a UTF-8 locale for python

build: force a UTF-8 locale for python

Python3 versions less than 3.7 have very unhelpful handling
of the C locale where they assume data is 7-bit only. This
violates POSIX which requires the C locale to be 8-bit clean.
Python3 >= 3.7 now assumes that the C locale is always UTF-8.

Set env variables to force LC_CTYPE to en_US.UTF-8 so that
we get UTF-8 handling on all python versions. Note we do
not use C.UTF-8 since not all C libraries support that.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>