bpo-40204, doc: Fix syntax of C variables (GH-21846)

For example, fix the following Sphinx 3 errors:

Doc/c-api/buffer.rst:102: WARNING: Error in declarator or parameters
Invalid C declaration: Expected identifier in nested name. [error at 5]
  void \*obj
  -----^

Doc/c-api/arg.rst:130: WARNING: Unparseable C cross-reference: 'PyObject*'
Invalid C declaration: Expected end of definition. [error at 8]
  PyObject*
  --------^

The modified documentation is compatible with Sphinx 2 and Sphinx 3.
diff --git a/Doc/c-api/exceptions.rst b/Doc/c-api/exceptions.rst
index b4722ff..247b6d6 100644
--- a/Doc/c-api/exceptions.rst
+++ b/Doc/c-api/exceptions.rst
@@ -783,7 +783,7 @@
 
 All standard Python exceptions are available as global variables whose names are
 ``PyExc_`` followed by the Python exception name.  These have the type
-:c:type:`PyObject\*`; they are all class objects.  For completeness, here are all
+:c:type:`PyObject*`; they are all class objects.  For completeness, here are all
 the variables:
 
 .. index::
@@ -1003,7 +1003,7 @@
 
 All standard Python warning categories are available as global variables whose
 names are ``PyExc_`` followed by the Python exception name. These have the type
-:c:type:`PyObject\*`; they are all class objects. For completeness, here are all
+:c:type:`PyObject*`; they are all class objects. For completeness, here are all
 the variables:
 
 .. index::