reorganize tox.ini a bit and use pytest-cov rather than coverage directly

tox.ini

@@ -21,12 +21,11 @@ deps = setuptools
 [testenv:py37]
 # run pytest and coverage
 deps =
-    coverage
     pytest
     pytest-cov
 commands =
-    pytest --cov={envsitepackagesdir}/incorporealcms/ --cov-report=
-    coverage report -m --fail-under=80
+    pytest --cov={envsitepackagesdir}/incorporealcms/ \
+           --cov-report=term-missing --cov-branch --cov-fail-under=90
     ln -sf {distdir} dist
 
 [testenv:security]
@@ -52,6 +51,16 @@ commands =
     flake8
     - flake8 --disable-noqa --select=E,W,F,C,D,A,G,B
 
+[coverage:paths]
+source =
+    ./
+    .tox/**/site-packages/
+
+[coverage:run]
+branch = True
+omit =
+    **/_version.py
+
 [flake8]
 max-line-length = 120
 exclude =
@@ -66,13 +75,3 @@ python_files =
     *_tests.py
     tests.py
     test_*.py
-
-[coverage:run]
-branch = True
-omit =
-    **/_version.py
-
-[coverage:paths]
-source =
-    ./
-    .tox/**/site-packages/