bpo-42260: Fix _PyConfig_Read() if compute_path_config=0 (GH-23220)

Fix _PyConfig_Read() if compute_path_config=0: use values set by
Py_SetPath(), Py_SetPythonHome() and Py_SetProgramName(). Add
compute_path_config parameter to _PyConfig_InitPathConfig().

The following functions now return NULL if called before
Py_Initialize():

* Py_GetExecPrefix()
* Py_GetPath()
* Py_GetPrefix()
* Py_GetProgramFullPath()
* Py_GetProgramName()
* Py_GetPythonHome()

These functions no longer automatically computes the Python Path
Configuration. Moreover, Py_SetPath() no longer computes
program_full_path.

Doc/c-api/init.rst

@@ -376,6 +376,12 @@ Process-wide parameters
    The returned string points into static storage; the caller should not modify its
    value.
 
+   This function should not be called before :c:func:`Py_Initialize`, otherwise
+   it returns ``NULL``.
+
+   .. versionchanged:: 3.10
+      It now returns ``NULL`` if called before :c:func:`Py_Initialize`.
+
 
 .. c:function:: wchar_t* Py_GetPrefix()
 
@@ -389,6 +395,12 @@ Process-wide parameters
    script at build time.  The value is available to Python code as ``sys.prefix``.
    It is only useful on Unix.  See also the next function.
 
+   This function should not be called before :c:func:`Py_Initialize`, otherwise
+   it returns ``NULL``.
+
+   .. versionchanged:: 3.10
+      It now returns ``NULL`` if called before :c:func:`Py_Initialize`.
+
 
 .. c:function:: wchar_t* Py_GetExecPrefix()
 
@@ -424,6 +436,12 @@ Process-wide parameters
    while having :file:`/usr/local/plat` be a different filesystem for each
    platform.
 
+   This function should not be called before :c:func:`Py_Initialize`, otherwise
+   it returns ``NULL``.
+
+   .. versionchanged:: 3.10
+      It now returns ``NULL`` if called before :c:func:`Py_Initialize`.
+
 
 .. c:function:: wchar_t* Py_GetProgramFullPath()
 
@@ -437,6 +455,12 @@ Process-wide parameters
    static storage; the caller should not modify its value.  The value is available
    to Python code as ``sys.executable``.
 
+   This function should not be called before :c:func:`Py_Initialize`, otherwise
+   it returns ``NULL``.
+
+   .. versionchanged:: 3.10
+      It now returns ``NULL`` if called before :c:func:`Py_Initialize`.
+
 
 .. c:function:: wchar_t* Py_GetPath()
 
@@ -455,8 +479,14 @@ Process-wide parameters
    can be (and usually is) modified later to change the search path for loading
    modules.
 
+   This function should not be called before :c:func:`Py_Initialize`, otherwise
+   it returns ``NULL``.
+
    .. XXX should give the exact rules
 
+   .. versionchanged:: 3.10
+      It now returns ``NULL`` if called before :c:func:`Py_Initialize`.
+
 
 .. c:function::  void Py_SetPath(const wchar_t *)
 
@@ -638,6 +668,12 @@ Process-wide parameters
    :c:func:`Py_SetPythonHome`, or the value of the :envvar:`PYTHONHOME`
    environment variable if it is set.
 
+   This function should not be called before :c:func:`Py_Initialize`, otherwise
+   it returns ``NULL``.
+
+   .. versionchanged:: 3.10
+      It now returns ``NULL`` if called before :c:func:`Py_Initialize`.
+
 
 .. _threads: