Pin dependencies for py36 compatibility [ussuri backport]

[freyes]

macaroonbakery in version 1.3.4 started depending on protobuf>3.20 which it's not compatible with python 3.6, so this change pins macaroonbakery to 1.3.2 which was the last release compatible with older versions of protobuf.

For more details see:

• https://github.com/go-macaroon-bakery/py-macaroon-bakery/pull/92
• https://github.com/protocolbuffers/protobuf/issues/10076

It's worth to mention that there is no protobuf-4.0, it goes from 3.20 to 4.21 - https://pypi.org/project/protobuf/#history

(cherry picked from commit a8e4de518f32503e788547e78ff2f609c0275dc3) (cherry picked from commit 300c5bbefb1228e20d53226951662074bbb729d4)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (8d58b5f) 82.27% compared to head (6e972ae) 82.27%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## stable/ussuri #636 +/- ## ============================================== Coverage 82.27% 82.27% ============================================== Files 30 30 Lines 2776 2776 ============================================== Hits 2284 2284 Misses 492 492 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.