Try to use Python3.7 on AppVeyor

Author: adamtheturtle

.appveyor.yml

@@ -1,5 +1,9 @@
+environment:
+  matrix:
+  - PYTHON: "C:\\Python37"
+
 install:
-- pip install --upgrade pip setuptools
-- pip install --upgrade --editable .[dev]
+- %PYTHON%\\python.exe -m pip install --upgrade pip setuptools
+- %PYTHON%\\python.exe -m pip install --upgrade --editable .[dev]
 test_script:
-- pytest -vvv --exitfirst tests/ --cov-fail-under 100 --cov src --cov tests
+- %PYTHON%\\python.exe -m pytest -vvv --exitfirst tests/ --cov-fail-under 100 --cov src --cov tests

setup.cfg

@@ -1,6 +1,7 @@
 [check-manifest]
 ignore =
     *.enc
+	.appveyor.yml
     .coveragerc
     .isort.cfg
     .markdownlint.json