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.

Doc/c-api/structures.rst

@@ -145,7 +145,7 @@ Implementing functions and methods
 .. c:type:: PyCFunction
 
    Type of the functions used to implement most Python callables in C.
-   Functions of this type take two :c:type:`PyObject\*` parameters and return
+   Functions of this type take two :c:type:`PyObject*` parameters and return
    one such value.  If the return value is ``NULL``, an exception shall have
    been set.  If not ``NULL``, the return value is interpreted as the return
    value of the function as exposed in Python.  The function must return a new
@@ -224,10 +224,10 @@ Implementing functions and methods
    +------------------+---------------+-------------------------------+
 
 The :attr:`ml_meth` is a C function pointer.  The functions may be of different
-types, but they always return :c:type:`PyObject\*`.  If the function is not of
+types, but they always return :c:type:`PyObject*`.  If the function is not of
 the :c:type:`PyCFunction`, the compiler will require a cast in the method table.
 Even though :c:type:`PyCFunction` defines the first parameter as
-:c:type:`PyObject\*`, it is common that the method implementation uses the
+:c:type:`PyObject*`, it is common that the method implementation uses the
 specific C type of the *self* object.
 
 The :attr:`ml_flags` field is a bitfield which can include the following flags.
@@ -239,7 +239,7 @@ There are these calling conventions:
 .. data:: METH_VARARGS
 
    This is the typical calling convention, where the methods have the type
-   :c:type:`PyCFunction`. The function expects two :c:type:`PyObject\*` values.
+   :c:type:`PyCFunction`. The function expects two :c:type:`PyObject*` values.
    The first one is the *self* object for methods; for module functions, it is
    the module object.  The second parameter (often called *args*) is a tuple
    object representing all arguments. This parameter is typically processed
@@ -260,7 +260,7 @@ There are these calling conventions:
    Fast calling convention supporting only positional arguments.
    The methods have the type :c:type:`_PyCFunctionFast`.
    The first parameter is *self*, the second parameter is a C array
-   of :c:type:`PyObject\*` values indicating the arguments and the third
+   of :c:type:`PyObject*` values indicating the arguments and the third
    parameter is the number of arguments (the length of the array).
 
    This is not part of the :ref:`limited API <stable>`.
@@ -274,7 +274,7 @@ There are these calling conventions:
    with methods of type :c:type:`_PyCFunctionFastWithKeywords`.
    Keyword arguments are passed the same way as in the
    :ref:`vectorcall protocol <vectorcall>`:
-   there is an additional fourth :c:type:`PyObject\*` parameter
+   there is an additional fourth :c:type:`PyObject*` parameter
    which is a tuple representing the names of the keyword arguments
    (which are guaranteed to be strings)
    or possibly ``NULL`` if there are no keywords.  The values of the keyword
@@ -312,7 +312,7 @@ There are these calling conventions:
    Methods with a single object argument can be listed with the :const:`METH_O`
    flag, instead of invoking :c:func:`PyArg_ParseTuple` with a ``"O"`` argument.
    They have the type :c:type:`PyCFunction`, with the *self* parameter, and a
-   :c:type:`PyObject\*` parameter representing the single argument.
+   :c:type:`PyObject*` parameter representing the single argument.
 
 
 These two constants are not used to indicate the calling convention but the
@@ -463,7 +463,7 @@ Accessing attributes of extension types
    |             |                  | getter and setter                 |
    +-------------+------------------+-----------------------------------+
 
-   The ``get`` function takes one :c:type:`PyObject\*` parameter (the
+   The ``get`` function takes one :c:type:`PyObject*` parameter (the
    instance) and a function pointer (the associated ``closure``)::
 
       typedef PyObject *(*getter)(PyObject *, void *);
@@ -471,7 +471,7 @@ Accessing attributes of extension types
    It should return a new reference on success or ``NULL`` with a set exception
    on failure.
 
-   ``set`` functions take two :c:type:`PyObject\*` parameters (the instance and
+   ``set`` functions take two :c:type:`PyObject*` parameters (the instance and
    the value to be set) and a function pointer (the associated ``closure``)::
 
       typedef int (*setter)(PyObject *, PyObject *, void *);