devel/py-pyinstaller: fix build with python 3.10

Update internal help2rst.py script to support output of from python
3.10 argparse

Approved by:	portmgr blanket

devel/py-pyinstaller/files/patch-doc_help2rst.py

+--- doc/help2rst.py.orig	2019-07-09 19:14:04 UTC
++++ doc/help2rst.py
+@@ -63,6 +63,9 @@ def process(program, generate_headings, headings_chara
+     elif '\noptional arguments:' in help:
+         # argparse style
+         help = help.split('\noptional arguments:', 1)[1]
++    elif '\noptions:' in help:
++        # argparse style, since python 3.10
++        help = help.split('\noptions:', 1)[1]
+     else:
+         raise SystemError('unexpected format of output for --help')
+     # Remove any obsolete options