Use sys.executable to determine --pycmd (#715)

* Use sys.executable to determine --pycmd

In some systems, `python` may not correctly refer to the virtual environment's `python` used for webui, or it even refers to Python 2.

Also in Windows, when the webui is run directly through `venv\Scripts\python` without activating the virtual environment, the system python will be picked instead of the one inside virtual environment.

* Remove reduntant "or".

config.py

@@ -1,4 +1,5 @@
 import argparse
+import sys
 import torch
 from multiprocessing import cpu_count
 
@@ -33,10 +34,11 @@ class Config:
 
     @staticmethod
     def arg_parse() -> tuple:
+        exe = sys.executable or "python"
         parser = argparse.ArgumentParser()
         parser.add_argument("--port", type=int, default=7865, help="Listen port")
         parser.add_argument(
-            "--pycmd", type=str, default="python", help="Python command"
+            "--pycmd", type=str, default=exe, help="Python command"
         )
         parser.add_argument("--colab", action="store_true", help="Launch in colab")
         parser.add_argument(