Initialization of Helpers._get_default_encoding_func is corrected [py3.9] (#221)

Python 3.9 does not undestand the following code:

_get_default_encoding_func = _make_get_default_encoding_func()

ERROR - TypeError: 'staticmethod' object is not callable

https://app.travis-ci.com/github/postgrespro/testgres/jobs/631402370

The code:
_get_default_encoding_func = _make_get_default_encoding_func.__func__()

is processed without problems.

Author: dmitry-lipetsk

testgres/operations/helpers.py

@@ -12,7 +12,7 @@ def _make_get_default_encoding_func():
         return locale.getpreferredencoding
 
     # Prepared pointer on function to get a name of system codepage
-    _get_default_encoding_func = _make_get_default_encoding_func()
+    _get_default_encoding_func = _make_get_default_encoding_func.__func__()
 
     @staticmethod
     def GetDefaultEncoding():