[1.6.x] Fixed #22551 -- Made oracle backend method Python 3 compatible

Thanks fatal10110 at gmail.com for the report. The fix is 1.6-only
because that code has been refactored in 1.7 (6983201cfb).
[1.6.x] Fixed #22558 -- Added missing word in contributing docs

Backport of c8d257e81 from master.
[1.6.x] Fixed #22565 -- Prevented pgettext_lazy crash with bytestring input

Thanks ygbo for the report.
Backport of 142c27218 from master.