Bug report

Bug summary
It appears that only integer sqrt bases are allowed by mathtext. The documentation does not list this limitation.

Code for reproduction

import matplotlib.pyplot as plt

fig = plt.figure()
axis = fig.add_subplot(1, 1, 1)
axis.plot(1, 1, label=r'$\sqrt[a]{{x}}$')
axis.legend()
plt.show()

Actual outcome
mathtext.Parser.parse raises a ValueError.

Expected outcome
The documentation states the following:

Radicals can be produced with the \sqrt[]{} command ... Any base can (optionally) be provided inside square brackets. Note that the base must be a simple expression, and can not contain layout commands such as fractions or sub/superscripts

In fact, it seems that only a (signed) integer literal is allowed. From mathtext.Parser.__init__:

p.sqrt          <<= Group(
                     Suppress(Literal(r"\sqrt"))
                   - ((Optional(p.lbracket + p.int_literal + p.rbracket, default=None)
                      + p.required_group)
                   | Error("Expected \sqrt{value}"))
                 )

It would be convenient if other bases were allowed. If that change cannot be made (or in the meantime), I think the documentation should be updated.

Matplotlib version

• Operating System: Debian 9.0
• Matplotlib Version: 2.0.0
• Python Version: 3.5.3

I believe I installed Python from the python3.5 package in the stretch repository and Matplotlib from the python3-matplotlib package in the stretch repository, but I'm not sure.