A string type compatibility issue on Python 3 resulted in a failure to

apply syntax highlighting using Pygments. After generating the

syntax-highlighted HTML (which is returned as a Unicode string), we

attempted to split the lines using split_line_endings() (which

required a byte string), and the type discrepency resulted in an

exception that was caught and ignored, causing the diff viewer to fall

back on a non-highlighted view.
To address this, split_line_endings() has been made compatible with

both byte and Unicode strings, and we no longer blindly ignore all

exceptions. Now, the only exception that is ignored is Pygments'

ClassNotFound, which is used when it can't determine a lexer for the

file type.
As part of the fix for exception handling, the code has been reworked to

both handle exceptions and to look up files in the highlighting

blacklist entirely within _apply_pygments(), to simplify the

requirements for the caller and to ensure that if the left-hand side of

the diff fails to syntax-highlight, it won't prevent the right-hand side

from trying.
Unit tests have been added to ensure that both split_line_endings()

and _apply_pygments() will do their jobs correctly, and that the final

chunk results from RawDiffChunkGenerator contain the resulting lines

we expect.

All unit tests passed on Python 2.7, 3.5, 3.6, and 3.7.
Verified that syntax highlighting was functioning properly.