[gdb/testsuite] Handle quotes in gdb_py_module_available

[gdb/testsuite] Handle quotes in gdb_py_module_available

On openSUSE Leap 42.3 with python 3.4, I run into:
...
(gdb) python import pygments^M
Traceback (most recent call last):^M
  File "<string>", line 1, in <module>^M
ImportError: No module named 'pygments'^M
Error while executing Python code.^M
(gdb) FAIL: gdb.base/style.exp: python import pygments
ERROR: unexpected output from python import
...
because gdb_py_module_available doesn't handle the single quotes around the
module name in the ImportError.

Fix this by allowing the single quotes.

Tested on x86_64-linux.