ci: simplify tox config

tox.ini

 [tox]
 isolated_build = true
-envlist = clean,lint,test,report
+envlist = lint,test
 
-[testenv]
+[testenv:lint]
 deps =
-    coverage
     flake8
-    pytest
-    pytest-cov
-
-[testenv:lint]
 commands =
     flake8 pycgtool
 
 [testenv:test]
+deps =
+    pytest
+    pytest-cov
 commands =
-    pytest --cov=pycgtool --cov-append test/
-setenv =
-    COVERAGE_FILE = .coverage.{envname}
-
-[testenv:report]
-skip_install = true
-commands =
-    coverage combine
-    coverage html
-    coverage report --fail-under=80
-depends = test
-
-[testenv:clean]
-skip_install = true
-commands =
-    coverage erase
+    pytest --cov=pycgtool --cov-fail-under=80 test/
 
 [flake8]
 max-line-length = 120