lib: try to find python after python3

[sam-github]

Unadorned python can be either Python 2 or Python 3, but it is likely
to be Python 2 for quite a while.

To find Python3, it is recommended to use the explicit name python3.

See:

• https://www.python.org/dev/peps/pep-0394/#for-python-runtime-distributors
• Release proposal: v6.0.0 #1892 (comment)

Checklist

• npm install && npm test passes
• tests are included
• documentation is changed or added
• commit message follows commit guidelines

Description of change

[sam-github]

I'm not sure what to do about

node-gyp/lib/find-python.js

Lines 186 to 210 in f36bd22

	// Check if the py launcher can find a valid Python to use.
	// Will exit the Python finder on success.
	// Distributions of Python on Windows by default install with the "py.exe"
	// Python launcher which is more likely to exist than the Python executable
	// being in the $PATH.
	// Because the Python launcher supports all versions of Python, we have to
	// explicitly request a Python 2 version. This is done by supplying "-2" as
	// the first command line argument. Since "py.exe -2" would be an invalid
	// executable for "execFile", we have to use the launcher to figure out
	// where the actual "python.exe" executable is located.
	checkPyLauncher: function checkPyLauncher (errorCallback) {
	this.log.verbose(
	`- executing "${this.pyLauncher}" to get Python 2 executable path`)
	this.run(this.pyLauncher, ['-2', ...this.argsExecutable], false,
	function (err, execPath) {
	// Possible outcomes: same as checkCommand
	if (err) {
	this.addLog(
	`- "${this.pyLauncher}" is not in PATH or produced an error`)
	return errorCallback(err)
	}
	this.addLog(`- executable path is "${execPath}"`)
	this.checkExecPath(execPath, errorCallback)
	}.bind(this))
	},

I don't have any Windows systems available ATM, so if I duplicate/refactor the Py Launcher stuff to look for 3 first, then 2, I've no way to test, unless its tested in CI, but I'm thinking it isn't.

I left it alone for now, though I did switch the order of the Windows default locations:

node-gyp/lib/find-python.js

Lines 29 to 30 in f36bd22

	path.join(process.env.SystemDrive || 'C:', 'Python27', 'python.exe'),
	path.join(process.env.SystemDrive || 'C:', 'Python37', 'python.exe')

[cclauss]

I agree with this order for v6.0.0:

1. python3
2. python
3. python2

However, the only acceptable Pythons are 2.7, 3.5, 3.6, and 3.7. So if python3 is Python 3.4 then we do not use it and we continue down the list.

[sam-github]

I note with relief that there are some Windows tests in Travis.

[joaocgreis]

@sam-github The change of order of default locations LGTM. You can simply move the PyLauncher to after the default location check, swap the blocks starting on lines 108 and 115. I plan to review the PyLauncher code for Python 3 sometime, and add registry detection like we have in core.

[sam-github]

@joaocgreis reordered as you suggested, which required some of the test monkey-patching to be adjusted for the new order of execFile()s.

[rvagg]

landed in dd0e97e, let's pull the trigger then eh?