Attempt to fix #11457

Attempt to fix #11457

In python 3 the colors need to be strings and not bytes:

This was the equivalent of

>> print("%s" % b"1")
"b'1'"

since the colors were bytes the call to dvipng was something like

dvipng -Ttight -depth -height -D 115 -fg "b'rgb 0.937255 0.941176 0.945098'" -bg "b'rgb 0.137255 0.149020 0.160784'"  "lyxpreviewxBJEqm.dvi"

Note the "b'rgb after both -fg and -bg that wrecked havoc and thus dvipng failed. That was the difference between python2 and python3 calls.

(cherry picked from commit 8dd31803b16d1fb6b30772955188c3595437f64a)