Prefer python3 command

[DanielThomas]

If version-less symlinks are present on the system (required for scripts with #!/usr/bin/env python for instance) assuming that python is Python 2 will fail. This prefers python3 before falling back to python.

[DanielThomas]

In fact it occurs to me that print() should be fine in any case, don't quite follow why there needs to be this distinction?

[matthewadams]

I've come to this because I can't get pyenv, installed via homebrew, to build python 2 on my M1 mac:

$ pyenv install 2.7.9
python-build: use openssl from homebrew
python-build: use readline from homebrew
Downloading Python-2.7.9.tar.xz...
-> https://www.python.org/ftp/python/2.7.9/Python-2.7.9.tar.xz
Installing Python-2.7.9...
patching file ./Lib/site.py
patching file ./Lib/ssl.py
python-build: use readline from homebrew
python-build: use zlib from xcode sdk

BUILD FAILED (OS X 12.3.1 using python-build 20180424)

Inspect or clean up the working tree at /var/folders/kt/g1jvgqy51tj1tpl7bbh1zd_c0000gn/T/python-build.20220503110231.54830
Results logged to /var/folders/kt/g1jvgqy51tj1tpl7bbh1zd_c0000gn/T/python-build.20220503110231.54830.log

Last 10 log lines:
checking size of _Bool... 1
checking for uintptr_t... yes
checking size of uintptr_t... 8
checking size of off_t... 8
checking whether to enable large file support... no
checking size of time_t... 8
checking for pthread_t... yes
checking size of pthread_t... 8
checking for --enable-toolbox-glue... yes
configure: error: Unexpected output of 'arch' on OSX

So, I have no python available on my path, only python3. Based on this PR, I updated line 185 of gng at /opt/homebrew/Cellar/gng/1.0.3/bin/gng to be

      python3 -c "import json,sys;obj=json.load(sys.stdin);print(obj['version']);") || die "Failed to fetch the version info!"

Everything worked fine. Please accept this MR. No reason not to, IMHO, especially since python version 2.x has been EOLed.