Bug#845047: python-guess-language FTBFS in stretch due to missing python3-3to2

[Adrian Bunk]

Source: python-guess-language
Version: 0.5.2-3
Severity: serious
Tags: stretch
Control: block -1 by 838974

python-guess-language build-depends on python3-3to2,
which is not in testing due to #838974