Not compatible with pyenv-win or other Python environments without PY.EXE

When I setup a new database with Python language on Windows, an exception was raised:

```
λ codeql database create talkbook --language=python --source-root=d:/pro/talkbook
Initializing database at d:\pro\codeql-repo\talkbook.
Running command [D:\program\codeql\python\tools\autobuild.cmd] in d:\pro\talkbook.
[2020-08-21 02:27:51] [build] Calling py -3 D:\program\codeql\python\tools\get_venv_lib.py
[2020-08-21 02:27:51] [build] Error trying to run get_venv_lib (this is Python 3.7.2)
[2020-08-21 02:27:51] [build] Traceback (most recent call last):
[2020-08-21 02:27:51] [build] File line 62, in lib
[2020-08-21 02:27:51] [build] self._lib = subprocess.check_output(args)
[2020-08-21 02:27:51] [build] File "C:\Users\phith0n\.pyenv\pyenv-win\versions\3.7.2\lib\subprocess.py", line 395, in check_output
[2020-08-21 02:27:51] [build] **kwargs).stdout
[2020-08-21 02:27:51] [build] File "C:\Users\phith0n\.pyenv\pyenv-win\versions\3.7.2\lib\subprocess.py", line 472, in run
[2020-08-21 02:27:51] [build] with Popen(*popenargs, **kwargs) as process:
[2020-08-21 02:27:51] [build] File "C:\Users\phith0n\.pyenv\pyenv-win\versions\3.7.2\lib\subprocess.py", line 775, in __init__
[2020-08-21 02:27:51] [build] restore_signals, start_new_session)
[2020-08-21 02:27:51] [build] File "C:\Users\phith0n\.pyenv\pyenv-win\versions\3.7.2\lib\subprocess.py", line 1178, in _execute_child
[2020-08-21 02:27:51] [build] startupinfo)
[2020-08-21 02:27:51] [build] FileNotFoundError: [WinError 2] The system cannot find the file specified
[2020-08-21 02:27:51] [build] Calling py -3 -S D:\program\codeql\python\tools\python_tracer.py -v -z all -c d:\pro\codeql-repo\talkbook\working\trap_cache -p d:\pro\codeql-repo\talkbook\working\venv\Lib -R d:\pro\talkbook
[2020-08-21 02:27:51] [build-err] Traceback (most recent call last):
[2020-08-21 02:27:51] [build-err] File "D:\program\codeql\python\tools\index.py", line 19, in
[2020-08-21 02:27:51] [build-err] buildtools.index.main()
[2020-08-21 02:27:51] [build-err] File "D:\program\codeql\python\tools\python3src.zip\buildtools\index.py", line 110, in main
[2020-08-21 02:27:51] [build-err] File "C:\Users\phith0n\.pyenv\pyenv-win\versions\3.7.2\lib\subprocess.py", line 342, in check_call
[2020-08-21 02:27:51] [build-err] retcode = call(*popenargs, **kwargs)
[2020-08-21 02:27:51] [build-err] File "C:\Users\phith0n\.pyenv\pyenv-win\versions\3.7.2\lib\subprocess.py", line 323, in call
[2020-08-21 02:27:51] [build-err] with Popen(*popenargs, **kwargs) as p:
[2020-08-21 02:27:51] [build-err] File "C:\Users\phith0n\.pyenv\pyenv-win\versions\3.7.2\lib\subprocess.py", line 775, in __init__
[2020-08-21 02:27:51] [build-err] restore_signals, start_new_session)
[2020-08-21 02:27:51] [build-err] File "C:\Users\phith0n\.pyenv\pyenv-win\versions\3.7.2\lib\subprocess.py", line 1178, in _execute_child
[2020-08-21 02:27:51] [build-err] startupinfo)
[2020-08-21 02:27:51] [build-err] FileNotFoundError: [WinError 2] The system cannot find the file specified
[2020-08-21 02:27:51] [ERROR] Spawned process exited abnormally (code 1; tried to run: [D:\program\codeql\tools\win64\runner.exe, cmd.exe, /C, type, NUL, &&, D:\program\codeql\python\tools\autobuild.cmd])
A fatal error occurred: Exit status 1 from command: [D:\program\codeql\tools\win64\runner.exe, cmd.exe, /C, type, NUL, &&, D:\program\codeql\python\tools\autobuild.cmd]
```

After review about the reason of this mess, I found that I don't have the `py.exe` executable program, which is used by `python\tools\python3src.zip\buildtools\version.py`:

```python
WIN = (sys.platform == 'win32')

def executable(version):
if WIN:
return ['py', ('-%s' % version)]
else:
return [('python%s' % version)]
```

I install the Python with [pyenv-win](https://github.com/pyenv-win/pyenv-win), which is a Python version management. Pyenv-win will not install the `py.exe` for users, maybe it was the cause of the exception.

Otherwise, **py.exe** is not a necessary thing for Python environment. So I believe there is a much better way to choice a right executable program rather than depending on **py.exe**.