Declare that parallel builds are supported

Populate and return extension metadata to declare that
sphinxext-argparse supports parallel builds. See sphinx documentation:
https://www.sphinx-doc.org/en/master/extdev/index.html?highlight=parallel_read_safe#extension-metadata

Fixes alex-rudakov/sphinx-argparse#105.

Author: lpsinger

sphinxarg/ext.py

@@ -12,6 +12,8 @@
 
 from sphinxarg.parser import parse_parser, parser_navigate
 
+from . import __version__
+
 
 def map_nested_definitions(nested_content):
     if nested_content is None:
@@ -510,3 +512,7 @@ def run(self):
 
 def setup(app):
     app.add_directive('argparse', ArgParseDirective)
+    return {
+        'parallel_read_safe': True,
+        'version': __version__
+    }