Add missing parentheses for python 3.10 compatibility

[jamesjer]

With python 3.10a6, the cvxopt build has started to fail, with errors such as this:

In file included from /usr/include/python3.10/Python.h:87,
                 from src/C/base.c:24:
src/C/base.c: In function ‘get_id’:
/usr/include/python3.10/object.h:242:38: error: expected ‘(’ before ‘_PyObject_TypeCheck’
  242 | #define PyObject_TypeCheck(ob, type) _PyObject_TypeCheck(_PyObject_CAST(ob), type)
      |                                      ^~~~~~~~~~~~~~~~~~~
src/C/cvxopt.h:73:28: note: in expansion of macro ‘PyObject_TypeCheck’
   73 | #define Matrix_Check(self) PyObject_TypeCheck(self, &matrix_tp)
      |                            ^~~~~~~~~~~~~~~~~~
src/C/base.c:356:8: note: in expansion of macro ‘Matrix_Check’
  356 |     if Matrix_Check((PyObject *)val)
      |        ^~~~~~~~~~~~
src/C/base.c:371:1: warning: control reaches end of non-void function [-Wreturn-type]
  371 | }
      | ^
error: command '/usr/bin/gcc' failed with exit code 1

Reminiscent of #164, the cause is an if expression that used to expand to a parenthesized expression, but no longer does.

[mkoeppe]

@martinandersen Any chance that this can be merged and a new release compatible with python 3.10 be cut?

[orlitzky]

FWIW the tentative python-3.10 release date is about three weeks from now: https://www.python.org/dev/peps/pep-0619/

[martinandersen]

@mkoeppe Sure, I'll see if I can get to it on Monday.