Fix pyinstaller compatibility (#1441)

Close https://github.com/zalando/patroni/issues/1440

patroni/__init__.py

@@ -169,9 +169,13 @@ class Patroni(object):
 
 def patroni_main():
     import argparse
+
+    from multiprocessing import freeze_support
     from patroni.config import Config, ConfigParseError
     from patroni.validator import schema
 
+    freeze_support()
+
     parser = argparse.ArgumentParser()
     parser.add_argument('--version', action='version', version='%(prog)s {0}'.format(__version__))
     parser.add_argument('--validate-config', action='store_true', help='Run config validator and exit')

patroni/dcs/__init__.py

@@ -66,8 +66,11 @@ def dcs_modules():
     module_prefix = __package__ + '.'
 
     if getattr(sys, 'frozen', False):
-        importer = pkgutil.get_importer(dcs_dirname)
-        return [module for module in list(importer.toc) if module.startswith(module_prefix) and module.count('.') == 2]
+        toc = set()
+        for importer in pkgutil.iter_importers(dcs_dirname):
+            if hasattr(importer, 'toc'):
+                toc |= importer.toc
+        return [module for module in toc if module.startswith(module_prefix) and module.count('.') == 2]
     else:
         return [module_prefix + name for _, name, is_pkg in pkgutil.iter_modules([dcs_dirname]) if not is_pkg]