Drop Python 3.6

Closes #53. Was sure this passed on GitHub actions,
but broken now and no reason to debug this further.

.github/workflows/test.yml

@@ -8,17 +8,13 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10"]
+        python-version: ["3.7", "3.8", "3.9", "3.10"]
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}
-    - name: Install old plaformdir
-      if: python-version == 3.6
-      run: |
-        pip install platformdir==2.4.0
     - name: Install package
       run: |
         python setup.py install

README.rst

@@ -178,6 +178,7 @@ Version Release date Changes
 v0.3.2  2022-02-25   - Use ``tomli`` library to match black, contribution from
 		       `Brian Helba <https://github.com/brianhelba>`_.
                      - Adopted GitHub Actions to replace TravisCI testing.
+                     - Python 3.7 or later required.
 v0.3.0  2022-02-25   - Requires black v22.1.0 (first non-beta release) or later.
                      - Support options "preview", "skip-magic-trailing-comma"
                        in the black TOML file, contribution from