Mark unvcrable tests that still need network

This is necessary to allow testing with --block-network -- just pass -m
'not novcr' as well to disable these tests.

Tests affected:

- tests/utils/test_ffmpeg.py::test_download_ffmpeg

  However, I doubt whether this function is even necessary -- shouldn't
  we be providing binaries with ffmpeg bundled instead for those users
  who can't install it conveniently on their machines?

pytest.ini

@@ -0,0 +1,3 @@
+[pytest]
+markers =
+    novcr: tests whose network traffic can't be cached

tests/utils/test_ffmpeg.py

@@ -101,6 +101,9 @@ def test_get_local_ffmpeg(monkeypatch):
         assert str(local_ffmpeg).endswith("ffmpeg.exe")
 
 
+# This can't be run with pytest --block-network without writing a 110M cassette
+# effectively containing the entire ffmpeg binary.
+@pytest.mark.novcr
 def test_download_ffmpeg(monkeypatch, tmpdir):
     """
     Test download_ffmpeg function.