Use a wrapper script for running emsdk.py (#292)

This change moves the python code for emsdk into a file ending in .py.
This script is then run via emsdk.bat on windows or emsdk (a shell
script) on non-windows.

This avoid the #!/bin/sh at the top of the python script and the "exec"
hack on the first line that re-runs it under python.  Hopefully this
preserves the intent of #273 without jumping through so many hoops.

.flake8

@@ -1,3 +1,3 @@
 [flake8]
 ignore = E111,E114,E501,E261,E266,E121,E402,E241,E701
-filename = emsdk
+filename = emsdk.py

emsdk.bat

@@ -33,7 +33,7 @@
 @SET EMSDK_PY=python
 
 :end
-@call %EMSDK_PY% "%~dp0\emsdk" %*
+@call %EMSDK_PY% "%~dp0\emsdk.py" %*
 
 @set EMSDK_PY=
 

python_selector

@@ -1,34 +0,0 @@
-#!/usr/bin/env python
-
-# Copyright 2019 The Emscripten Authors.  All rights reserved.
-# Emscripten is available under two separate licenses, the MIT license and the
-# University of Illinois/NCSA Open Source License.  Both these licenses can be
-# found in the LICENSE file.
-
-"""Provides a way to run a script on the preferred Python version"""
-
-import os
-import subprocess
-import sys
-
-
-def which(program):
-  for path in [""] + os.environ.get("PATH", "").split(os.pathsep):
-    exe_file = os.path.join(path, program)
-    if os.path.isfile(exe_file) and os.access(exe_file, os.X_OK):
-      # in case of pyenv we might have `python3` command available, but it's just a stub
-      # https://github.com/emscripten-core/emscripten/issues/8792
-      p = subprocess.Popen([exe_file, "--version"], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-      p.communicate()
-      if p.returncode == 0:
-        return exe_file
-  return None
-
-# Look for the best choice for python, favours Python3 over Python2
-# In case of Windows it uses always python that was used for this script
-if sys.platform in ["linux", "linux2", "darwin"]:
-  python = which("python3") or which("python") or sys.executable
-else:
-  python = sys.executable
-
-sys.exit(subprocess.call([python] + sys.argv[1:]))

test.py

@@ -33,7 +33,7 @@ def failing_call_with_output(cmd, expected):
   assert expected in stdout, 'call did not have the right output'
 
 def hack_emsdk(marker, replacement):
-  src = open('emsdk').read()
+  src = open('emsdk.py').read()
   assert marker in src
   src = src.replace(marker, replacement)
   name = '__test_emsdk'
@@ -89,7 +89,7 @@ print('update')
 check_call('./emsdk update-tags')
 
 print('test latest-releases-upstream')
-check_call('python2 ./emsdk install latest-upstream')
+check_call('python2 ./emsdk.py install latest-upstream')
 check_call('./emsdk activate latest-upstream')
 test_lib_building('upstream', use_asmjs_optimizer=False)
 assert open(os.path.expanduser('~/.emscripten')).read().count('LLVM_ROOT') == 1