tox| Uses -m coverage instead of path to site-packages (#3464)

As this was breaking on Fedora, use the coverage module invocation
instead of calling the site-package directly.

tox.ini

@@ -43,7 +43,7 @@ setenv =
     COVERAGE_FILE = {toxinidir}/.coverage.{envname}
 
 commands =
-    python -Wi {envsitepackagesdir}/coverage run -m pytest {toxinidir}/tests/ {posargs:}
+    python -Wignore -m coverage run -m pytest {toxinidir}/tests/ {posargs:}
 
     ; Transform absolute path to relative path
     ; for compatibility with coveralls.io and fix 'source not available' error.
@@ -74,8 +74,8 @@ deps =
     coveralls
 skip_install = true
 commands =
-    python {envsitepackagesdir}/coverage combine
-    python {envsitepackagesdir}/coverage report --rcfile={toxinidir}/.coveragerc -m
+    python -m coverage combine
+    python -m coverage report --rcfile={toxinidir}/.coveragerc -m
     - coveralls --rcfile={toxinidir}/.coveragerc
 changedir = {toxinidir}
 
@@ -86,7 +86,7 @@ deps =
     coverage<5
 skip_install = true
 commands =
-    python {envsitepackagesdir}/coverage erase
+    python -m coverage erase
 changedir = {toxinidir}
 
 [testenv:docs]