Rollup merge of #137493 - nabijaczleweli:python3, r=Kobzol

configure.py: don't instruct user to run nonexistent program

```shell-session
$ ./configure
configure: processing command line
configure:
configure: build.configure-args := []
configure: profile              := dist
configure:
configure: writing `config.toml` in current directory
configure:
configure: run `python /mnt/filling/store/nabijaczleweli/code/rust/x.py --help`
```

This is naturally not valid since I don't have a "python" executable (and this will hopefully become more and more true as Python 2 dies out).

./configure knows this since it does `try python3 "$``@"`,`` then `python2.7` &c.

After, this now says
```
configure: run `python3 /mnt/filling/store/nabijaczleweli/code/rust/x.py --help`
```
which is possible, and corresponds to the interpreter actually running.

Author: tgross35