2to3: Apply `repr` fixer.

This replaces python backtics with repr(...). The backtics were mostly
used to generate strings for printing with a string format and it is
tempting to replace `'%s' % repr(x)` with `'%r' % x`. That would work
except where `x` happened to be a tuple or a dictionary but, because it
would be significant work to guarantee that and because there are not
many places where backtics are used, the safe path is to let the repr
replacements stand.

Closes #3083.