Fix unaccent generation script in Windows

Fix unaccent generation script in Windows

As originally coded, the script would fail on Windows 10 and Python 3
because stdout would not be switched to UTF-8 only for Python 2.  This
patch makes that apply to both versions.

Also add python 2 compatibility markers so that we know what to remove
once we drop support for that.  Also use a "with" clause to ensure file
descriptor is closed promptly.

Author: Hugh Ranalli, Ramanarayana
Reviewed-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/CAKm4Xs7_61XMyOWmHs3n0mmkS0O4S0pvfWk=7cQ5P0gs177f7A@mail.gmail.com
Discussion: https://postgr.es/m/15548-cef1b3f8de190d4f@postgresql.org