Fix formatting for tick marks that are latexed

[kcrisman]

See ​this ask.sagemath question.

plot(1-4*x^2,.49999,.50001,tick_formatter=.00001.n(digits=7))

triggers the "if getting something in SR for a formatter then make sure both LaTeX" branch of the Graphics.matplotlib code and somehow that breaks with errors like

ValueError: 
\text{\texttt{{-}8e{-}05}}
^
Unknown symbol: \ttext (at char 0), (line:1, col:1)

so that mpl, in trying to typeset -8e^-5 somehow decides to prepend yet another text thing on it.