Run mpv subprocess with a system PYTHONPATH env on macOS.

youtube-dl relies on system python to run on macOS, but system
python cannot be executed in a subprocess created from the frozen
python3 executable. This makes youtube-dl unusable from frozen
versions of Syncplay on macOS. So, the environment of the mpv
subprocess is modified with system PYTHONPATH, allowing the
execution of /usr/bin/python (and hence, of youtube-dl)
from within the subprocess in frozen executables.

Fixes: Syncplay#228

syncplay/players/mplayer.py

@@ -1,4 +1,5 @@
 # coding:utf8
+import ast
 import os
 import re
 import subprocess
@@ -10,7 +11,7 @@
 from syncplay import constants, utils
 from syncplay.players.basePlayer import BasePlayer
 from syncplay.messages import getMessage
-from syncplay.utils import isWindows
+from syncplay.utils import isMacOS, isWindows
 
 
 class MplayerPlayer(BasePlayer):
@@ -338,6 +339,20 @@ def __init__(self, playerController, playerPath, filePath, args):
             env = os.environ.copy()
             if 'TERM' in env:
                 del env['TERM']
+            # On macOS, youtube-dl requires system python to run. Set the environment
+            # to allow that version of python to be executed in the mpv subprocess.
+            if isMacOS():
+                try:
+                    pythonLibs = subprocess.check_output(['/usr/bin/python', '-E', '-c',
+                                                          'import sys; print(sys.path)'],
+                                                          text=True, env=dict())
+                    pythonLibs = ast.literal_eval(pythonLibs)
+                    pythonPath = ':'.join(pythonLibs[1:])
+                except:
+                    pythonPath = None
+                if pythonPath is not None:
+                    env['PATH'] = '/usr/bin:/usr/local/bin'
+                    env['PYTHONPATH'] = pythonPath
             if filePath:
                 self.__process = subprocess.Popen(
                     call, stdout=subprocess.PIPE, stdin=subprocess.PIPE, stderr=subprocess.STDOUT,