gpython: Add support for -V / --version

On CPython:

	$ python -V
	Python 2.7.18

	$ gpython -V
	GPython 0.0.6.post2 [gevent 20.6.2] / CPython 2.7.18

On PyPy:

	$ python -V
        Python 3.6.9 (2ad108f17bdb, Apr 07 2020, 02:59:05)
        [PyPy 7.3.1 with GCC 7.3.1 20180303 (Red Hat 7.3.1-5)]

	$ gpython -V
	GPython 0.0.6.post2 [gevent 20.5.0] / PyPy 7.3.1 / Python 3.6.9

/reviewed-on !5

gpython/__init__.py

@@ -70,8 +70,37 @@ def pymain(argv):
         console.interact()
         return
 
+    # -V / --version
+    if argv[0] in ('-V', '--version'):
+        ver = []
+        if 'GPython' in sys.version:
+            golang = sys.modules['golang'] # must be already imported
+            gevent = sys.modules['gevent'] # must be already imported
+            gpyver = 'GPython %s [gevent %s]' % (golang.__version__, gevent.__version__)
+            ver.append(gpyver)
+
+        import platform
+        pyimpl = platform.python_implementation()
+
+        v = _version_info_str
+        if pyimpl == 'CPython':
+            ver.append('CPython %s' % v(sys.version_info))
+        elif pyimpl == 'PyPy':
+            ver.append('PyPy %s'   % v(sys.pypy_version_info))
+            ver.append('Python %s' % v(sys.version_info))
+        else:
+            ver = [] # unknown
+
+        ver = ' / '.join(ver)
+        if ver == '':
+            # unknown implementation: just print full sys.version
+            ver = sys.version
+
+        print(ver, file=sys.stderr)
+
+
     # -c command
-    if argv[0].startswith('-c'):
+    elif argv[0].startswith('-c'):
         cmd  = argv[0][2:] # -c<command> also works
         argv = argv[1:]
         if cmd == '':
@@ -127,6 +156,14 @@ def _execfile(path, globals=None, locals=None):
     code = compile(src, path, 'exec')
     six.exec_(code, globals, locals)
 
+# _version_info_str converts version_info -> str.
+def _version_info_str(vi):
+    major, minor, micro, release, serial = vi
+    v = '%d.%d.%d' % (major, minor, micro)
+    if (release, serial) != ('final', 0):
+        v += '.%s%s' % (release, serial)
+    return v
+
 
 def main():
     # set UTF-8 as default encoding.

gpython/gpython_test.py

@@ -20,12 +20,16 @@
 
 from __future__ import print_function, absolute_import
 
-import sys, os, golang
-from golang.golang_test import pyout
+import sys, os, platform, golang
+from golang.golang_test import pyout, _pyrun
+from subprocess import PIPE
 from six import PY2
 from six.moves import builtins
 import pytest
 
+is_pypy    = (platform.python_implementation() == 'PyPy')
+is_cpython = (platform.python_implementation() == 'CPython')
+
 # @gpython_only is marker to run a test only under gpython
 gpython_only = pytest.mark.skipif('GPython' not in sys.version, reason="gpython-only test")
 
@@ -130,3 +134,27 @@ def test_pymain():
     # file
     _ = pyout(['testdata/hello.py', 'abc', 'def'], cwd=here)
     assert _ == b"hello\nworld\n['testdata/hello.py', 'abc', 'def']\n"
+
+# pymain -V/--version
+# gpython_only because output differs from !gpython.
+@gpython_only
+def test_pymain_ver():
+    from golang import b
+    from gpython import _version_info_str as V
+    import gevent
+    vok = 'GPython %s [gevent %s]' % (golang.__version__, gevent.__version__)
+
+    if is_cpython:
+        vok += ' / CPython %s' % platform.python_version()
+    elif is_pypy:
+        vok += ' / PyPy %s / Python %s' % (V(sys.pypy_version_info), V(sys.version_info))
+    else:
+        vok = sys.version
+
+    vok += '\n'
+
+    ret, out, err = _pyrun(['-V'], stdout=PIPE, stderr=PIPE)
+    assert (ret, out, b(err)) == (0, b'', b(vok))
+
+    ret, out, err = _pyrun(['--version'], stdout=PIPE, stderr=PIPE)
+    assert (ret, out, b(err)) == (0, b'', b(vok))