python3Packages.pyflakes: disable failing test for PyPy3

test_misencodedFileUTF16 fails with PyPy3 as it outputs a different
error message than CPython.

Changed files
+1
pkgs
development
python-modules
pyflakes
+1
pkgs/development/python-modules/pyflakes/default.nix
···
disabledTests = lib.optionals isPyPy [
# https://github.com/PyCQA/pyflakes/issues/779
"test_eofSyntaxError"
+
"test_misencodedFileUTF16"
"test_misencodedFileUTF8"
"test_multilineSyntaxError"
];