gh-131189: Fix "msvcrt" import warning on Linux when "_ctypes" is not available.

[plashchynski]

On Linux, compiling without "libffi" causes a misleading warning
"No module named 'msvcrt'" when launching PyREPL.

• Issue: Compilation on Linux without both libffi-dev and libncurses-dev results in an unexpected warning when using the new REPL #131189

[bedevere-app]

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If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[sobolevn]

I cannot recreate the same setup as you have, can you please share what happens with this change in place?

[plashchynski]

@sobolevn steps to reproduce on a fresh Ubuntu Server 24.04.2 setup:

sudo apt install build-essential
git clone https://github.com/python/cpython.git
cd cpython
./configure
make

Then:

./python

shows:

Python 3.14.0a5+ (heads/main:c497f83ad85, Mar 14 2025, 09:42:58) [GCC 13.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
warning: can't use pyrepl: No module named 'msvcrt'
>>> 

Here's a full log.

with changes in this PR, it shows the following with the same setup:

Python 3.14.0a5+ (heads/fix_msvcrt_warning_on_linux:56364cef29, Mar 14 2025, 09:49:25) [GCC 13.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
warning: can't use pyrepl: No module named '_ctypes'
>>> 

Here's a full log.

warning: can't use pyrepl: No module named 'msvcrt'

this warning is misleading, because msvcrt is available only on NT systems.

warning: can't use pyrepl: No module named '_ctypes'

this warning is consistent with the warning make shows for missing necessary bits:

The necessary bits to build these optional modules were not found:
_bz2                      _ctypes                   _ctypes_test           
_curses                   _curses_panel             _dbm                   
_gdbm                     _hashlib                  _lzma                  
_ssl                      _tkinter                  _uuid                  
readline                  zlib                                             
To find the necessary bits, look in configure.ac and config.log.

[sobolevn]

Thanks, it is clear now! LGTM. However, I am not the module's maintainer, let's wait for them to make the final decision :)

[vstinner]

LGTM

[miss-islington-app]

Thanks @plashchynski for the PR, and @ambv for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

[bedevere-app]

GH-136668 is a backport of this pull request to the 3.14 branch.

[miss-islington-app]

Thanks @plashchynski for the PR, and @ambv for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

[bedevere-app]

GH-138934 is a backport of this pull request to the 3.13 branch.