Drop support for python 3.6 & improve tests

- Update dev dependencies, including poetry in poetry plugin tests
- Remove tox from config
- Refactor tests to reduce external dependencies

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2
         with:
-          python-version: 3.6
+          python-version: 3.7
       - uses: actions/cache@v2
         with:
           path: ~/.cache/pypoetry/virtualenvs
@@ -50,7 +50,7 @@ jobs:
     strategy:
       matrix:
         os: [Ubuntu, MacOS, Windows]
-        python-version: ['3.6', '3.7', '3.8', '3.9']
+        python-version: ['3.7', '3.8', '3.9', '3.10']
 
     steps:
       - uses: actions/checkout@v2