Avoid broken macaroonbakery release

macaroonbakery 1.3.3 has a broken protobuf dependency, add a requirement
to avoid that specific release:
go-macaroon-bakery/py-macaroon-bakery#94

Conflicts:
    test-requirements.txt

Also drop py36 from github workflow as it is no longer available
on ubuntu-latest and not needed for xena which is focal based.

(cherry picked from commit 8be4948)

.github/workflows/tox.yaml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.7, 3.8, 3.9]
 
     steps:
     - uses: actions/checkout@v1

requirements.txt

@@ -41,3 +41,5 @@ python-libmaas
 # Documentation requirements
 sphinx
 sphinxcontrib-asyncio
+# https://github.com/go-macaroon-bakery/py-macaroon-bakery/issues/94
+macaroonbakery!=1.3.3

setup.py

@@ -41,6 +41,9 @@
     'PyYAML',
     'tenacity',
     'python-libmaas',
+
+    # https://github.com/go-macaroon-bakery/py-macaroon-bakery/issues/94
+    'macaroonbakery != 1.3.3',
 ]
 
 tests_require = [