gdb/testsuite: add missing skip_python_tests call in py-doc-reformat.exp

In commit:

  commit 51e8dbe1fbe7d8955589703140ca5eba7b4f1bd7
  Date:   Mon May 16 19:26:54 2022 +0100

      gdb/python: improve formatting of help text for user defined commands

the test that was added (gdb.python/py-doc-reformat.exp) was missing a
call to skip_python_tests.  As a result, this test would fail for any
GDB built within Python support.

This commit adds a call to skip_python_tests.