tox: Update coverage to 4.0.1.

- When combining data files, unreadable files will now generate a warning
  instead of failing the command.  This is more in line with the older
  coverage.py v3.7.1 behavior, which silently ignored unreadable files.

- The --skip-covered option would skip reporting on 100% covered files, but
  also skipped them when calculating total coverage.  This was wrong, it should
  only remove lines from the report, not change the final answer.  This is now
  fixed.

- In 4.0, the data file recorded a summary of the system on which it was run.
  Combined data files would keep all of those summaries.  This could lead to
  enormous data files consisting of mostly repetitive useless information. That
  summary is now gone.  If you want summary information,
  get in touch, and we'll figure out a better way to do it.

- Test suites that mocked os.path.exists would experience strange failures, due
  to coverage.py using their mock inadvertently.  This is now fixed.

- Importing a ``__init__`` module explicitly would lead to an error:
  ``AttributeError: 'module' object has no attribute '__path__'``.  This is now
  fixed.

- Code that uses ``sys.settrace(sys.gettrace())`` used to incur a more than 2x
  speed penalty.  Now there's no penalty at all.

- Pyexpat C code will no longer be recorded as a source file.

- The source kit now contains all of the files needed to have a complete source
  tree.

tox.ini

@@ -25,7 +25,7 @@ deps =
     pytest-sugar==0.5.1
     xvfbwrapper==0.2.5
     hypothesis==1.11.4
-    coverage==4.0.0
+    coverage==4.0.1
     pytest-cov==2.2.0
     beautifulsoup4==4.4.1
     httpbin==0.3.0