tox.ini: require coverage>=4.3

Coverage 4.3 was released, it includes this patch:
https://bitbucket.org/ned/coveragepy/pull-requests/118/check-source-isdir-not-just-exists-there/diff

tox.ini

@@ -9,9 +9,7 @@ basepython =
     py35: {env:TOXPYTHON:python3.5}
     py36: {env:TOXPYTHON:python3.6}
 deps =
-    # we install coverage from source until 4.3 is released, because of this:
-    # https://bitbucket.org/ned/coveragepy/pull-requests/118/check-source-isdir-not-just-exists-there/diff
-    cov: hg+https://bitbucket.org/ned/coveragepy#egg=coverage
+    cov: coverage>=4.3
     pytest
     -rrequirements.txt
 install_command =
@@ -27,7 +25,7 @@ commands =
 [testenv:htmlcov]
 basepython = {env:TOXPYTHON:python3.5}
 deps =
-    hg+https://bitbucket.org/ned/coveragepy#egg=coverage
+    coverage>=4.3
 skip_install = true
 commands =
     coverage combine
@@ -37,7 +35,7 @@ commands =
 passenv = *
 basepython = {env:TOXPYTHON:python}
 deps =
-    hg+https://bitbucket.org/ned/coveragepy#egg=coverage
+    coverage>=4.3
     codecov
 skip_install = true
 ignore_outcome = true