[1.10.x] Fixed #26674 -- Corrected a i18n makemessages test.

Made it consistently read the PO file, decode its contents and then
check for the non-breaking space Unicode code point.

Previously we were erroneously skipping the interpretation of what we
read as UTF-8 text.

This was causing the test to fail on Windows with Python 3.5.

Backport of 1b00ed08 from master