Merge branch 4.x

modules/python/package/cv2/__init__.py

@@ -18,6 +18,10 @@ except ImportError:
 
 def bootstrap():
     import sys
+
+    import copy
+    save_sys_path = copy.copy(sys.path)
+
     if hasattr(sys, 'OpenCV_LOADER'):
         print(sys.path)
         raise ImportError('ERROR: recursion is detected during loading of "cv2" binary extensions. Check OpenCV installation.')
@@ -85,6 +89,8 @@ def bootstrap():
     del sys.modules['cv2']
     import cv2
 
+    sys.path = save_sys_path  # multiprocessing should start from bootstrap code (https://github.com/opencv/opencv/issues/18502)
+
     try:
         import sys
         del sys.OpenCV_LOADER

modules/python/package/setup.py

@@ -45,6 +45,7 @@ def main():
           'Programming Language :: Python :: 3.6',
           'Programming Language :: Python :: 3.7',
           'Programming Language :: Python :: 3.8',
+          'Programming Language :: Python :: 3.9',
           'Programming Language :: C++',
           'Programming Language :: Python :: Implementation :: CPython',
           'Topic :: Scientific/Engineering',