--source fails without python2 on Python 3

[ekalinin]

build_node_from_src requires a python2 command whenever nodeenv runs under Python 3:

if getattr(sys.version_info, 'major', sys.version_info[0]) > 2:
    # Currently, the node.js build scripts are using python2.*,
    # therefore we need to temporarily point python exec to the
    # python 2.* version in this case.
    python2_path = shutil.which('python2')
    if not python2_path:
        raise OSError(
            'Python >=3.0 virtualenv detected, but no python2 '
            'command (required for building node.js) was found'
        )

nodeenv is Python 3 only now, so this branch is always taken. Current node.js build scripts use Python 3, and python2 is usually absent on modern systems, so nodeenv --source fails before ./configure runs.

Reproduce

On master (e745358), on a machine without python2:

import types, nodeenv
args = types.SimpleNamespace(load_average=None, jobs='2', without_ssl=False,
    debug=False, profile=False, make_path='make', verbose=False)
nodeenv.build_node_from_src('/tmp/x/env', '/tmp/x', '/tmp/x/node-src', args)

OSError: Python >=3.0 virtualenv detected, but no python2 command (required for building node.js) was found