Prepare code and CI for Python 3.8 (#1399)

* Refactored get_collection_coverage tests

Using pytest's parametrize mark. This makes it easier to isolate test cases in order to fix the errors that are showing up when running under Python3.8

* Refactor tests from test_api.py that were failing under Python 3.8

* Fix flake8 issues

* Add Python 3.8 to the CI matrix

* Replaced pandas usage with numpy for datetime

* Reverted tests back to non-parametrized form

* Dropped Python 3.7 from main github CI workflow

---------

Co-authored-by: Ricardo Garcia Silva <ricardo@kartoza.com>

.github/workflows/main.yml

@@ -20,7 +20,7 @@ jobs:
       - name: Setup Python
         uses: actions/setup-python@v1
         with:
-          python-version: 3.7
+          python-version: 3.8
           architecture: x64
       - name: Checkout pygeoapi
         uses: actions/checkout@master
@@ -39,7 +39,7 @@ jobs:
     strategy:
       matrix:
         include:
-          - python-version: 3.7
+          - python-version: 3.8
     env:
       PYGEOAPI_CONFIG: "$(pwd)/pygeoapi-config.yml"