pythongh-108051: Update versions found by find_python.bat and clarify readme (pythonGH-116118)

Author: zooba

PCbuild/find_python.bat

@@ -29,6 +29,9 @@
 :begin_search
 @set PYTHON=
 
+@rem If PYTHON_FOR_BUILD is set, use that
+@if NOT "%PYTHON_FOR_BUILD%"=="" @(set PYTHON="%PYTHON_FOR_BUILD%") && (set _Py_Python_Source=found as PYTHON_FOR_BUILD) && goto :found
+
 @rem If there is an active virtual env, use that one
 @if NOT "%VIRTUAL_ENV%"=="" (set PYTHON="%VIRTUAL_ENV%\Scripts\python.exe") & (set _Py_Python_Source=found in virtual env) & goto :found
 
@@ -42,7 +45,9 @@
 @if NOT "%HOST_PYTHON%"=="" @%HOST_PYTHON% -Ec "import sys; assert sys.version_info[:2] >= (3, 9)" >nul 2>nul && (set PYTHON="%HOST_PYTHON%") && (set _Py_Python_Source=found as HOST_PYTHON) && goto :found
 
 @rem If py.exe finds a recent enough version, use that one
-@for %%p in (3.11 3.10 3.9) do @py -%%p -EV >nul 2>&1 && (set PYTHON=py -%%p) && (set _Py_Python_Source=found %%p with py.exe) && goto :found
+@rem It is fine to add new versions to this list when they have released,
+@rem but we do not use prerelease builds here.
+@for %%p in (3.12 3.11 3.10 3.9) do @py -%%p -EV >nul 2>&1 && (set PYTHON=py -%%p) && (set _Py_Python_Source=found %%p with py.exe) && goto :found
 
 @if NOT exist "%_Py_EXTERNALS_DIR%" mkdir "%_Py_EXTERNALS_DIR%"
 @set _Py_NUGET=%NUGET%

PCbuild/readme.txt

@@ -226,12 +226,18 @@ directory.  This script extracts all the external sub-projects from
 and
     https://github.com/python/cpython-bin-deps
 via a Python script called "get_external.py", located in this directory.
-If Python 3.6 or later is not available via the "py.exe" launcher, the
-path or command to use for Python can be provided in the PYTHON_FOR_BUILD
-environment variable, or get_externals.bat will download the latest
-version of NuGet and use it to download the latest "pythonx86" package
-for use with get_external.py.  Everything downloaded by these scripts is
-stored in ..\externals (relative to this directory).
+Everything downloaded by these scripts is stored in ..\externals
+(relative to this directory), or the path specified by the EXTERNALS_DIR
+environment variable.
+
+The path or command to use for Python can be provided with the
+PYTHON_FOR_BUILD environment variable. If this is not set, an active
+virtual environment will be used. If none is active, and HOST_PYTHON is
+set to a recent enough version or "py.exe" is able to find a recent
+enough version, those will be used. If all else fails, a copy of Python
+will be downloaded from NuGet and extracted to the externals directory.
+This will then be used for later builds (see PCbuild/find_python.bat
+for the full logic).
 
 It is also possible to download sources from each project's homepage,
 though you may have to change folder names or pass the names to MSBuild