Allow attributes in Configure instances to be set to none value before assigned a value

setup.py

@@ -62,9 +62,10 @@ def run_tests(self):
     classifiers=[
         "Development Status :: 4 - Beta",
         "License :: OSI Approved :: Apache Software License",
-        "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
+        "Programming Language :: Python :: 3.9",
+        "Programming Language :: Python :: 3.10",
         "Topic :: Software Development :: Libraries :: Python Modules"],
     install_requires=[
         "cryptojwt==1.6.1",

src/oidcmsg/__init__.py

@@ -1,5 +1,5 @@
 __author__ = "Roland Hedberg"
-__version__ = "1.5.4"
+__version__ = "1.6.0"
 
 import os
 from typing import Dict

src/oidcmsg/configure.py

@@ -105,12 +105,12 @@ def __getattr__(self, item, default=None):
             return default
 
     def __setattr__(self, key, value):
-        if key in self:
+        if key in self and self.key:
             raise KeyError('{} has already been set'.format(key))
         super(Base, self).__setitem__(key, value)
 
     def __setitem__(self, key, value):
-        if key in self:
+        if key in self and self.key:
             raise KeyError('{} has already been set'.format(key))
         super(Base, self).__setitem__(key, value)
 

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py{36,37,38},docs,quality
+envlist = py{37,38,39,310},docs,quality
 
 [testenv]
 passenv = CI TRAVIS TRAVIS_*