use coverage directly instead of through pytest-cov plugin

works better with parallel coverage

.coveragerc

@@ -2,7 +2,6 @@
 # measure 'branch' coverage in addition to 'statement' coverage
 # See: http://coverage.readthedocs.org/en/coverage-4.0.3/branch.html#branch
 branch = True
-parallel = True
 omit = tests/*
 
 # list of directories or packages to measure

test_requirements.txt

@@ -1,3 +1,3 @@
 -r requirements.txt
 pytest>=4.5.0
-pytest-cov>=2.7
+coverage>=5

tox.ini

@@ -1,17 +1,14 @@
-# More info on pytest-cov integration: https://pytest-cov.readthedocs.io/en/latest/tox.html
-# Coverage is intentionally not appended to each run because we do not have a coverage
-# service set up, it should be used by the developer for local runs only.
-
 [tox]
-envlist = lint, py{36,37}-cov, coverage-report
+envlist = lint, py3{6,7,8,9}, coverage-report
+skip_missing_interpreters = true
 
 [testenv]
 deps =
     -r test_requirements.txt
 ; download the latest pip, setuptools and wheel when creating the venv
 download = true
 commands =
-    pytest --cov fontmake {posargs}
+    coverage run --parallel-mode -m pytest {posargs}
 
 [testenv:lint]
 skip_install = true
@@ -23,4 +20,7 @@ commands =
 [testenv:coverage-report]
 skip_install = true
 commands =
+    coverage combine
+    coverage xml
+    coverage report
     coverage html