Add support for Python 3.10 + increase coverage for Python >= 3.7.4.

CHANGES.rst

@@ -7,6 +7,7 @@
 
 - Switch package to src-layout, this is a packaging only change.
   (`#168 <https://github.com/zopefoundation/persistent/pull/168>`_)
+- Add support for Python 3.10.
 
 
 4.7.0 (2021-04-13)

setup.py

@@ -101,6 +101,7 @@ def _read_file(filename):
           "Programming Language :: Python :: 3.7",
           "Programming Language :: Python :: 3.8",
           "Programming Language :: Python :: 3.9",
+          "Programming Language :: Python :: 3.10",
           "Programming Language :: Python :: Implementation :: CPython",
           "Programming Language :: Python :: Implementation :: PyPy",
           "Framework :: ZODB",

src/persistent/list.py

@@ -55,7 +55,7 @@ class PersistentList(UserList, persistent.Persistent):
         else lambda inst: inst.__delitem__(_SLICE_ALL)
     )
 
-    if not PYTHON2 and sys.version_info[:3] < (3, 7, 4):
+    if not PYTHON2 and sys.version_info[:3] < (3, 7, 4):  # pragma: no cover
         # Prior to 3.7.4, Python 3 (but not Python 2) failed to properly
         # return an instance of the same class.
         # See https://bugs.python.org/issue27639
@@ -68,7 +68,7 @@ def __getitem__(self, item):
                 result = self.__class__(result)
             return result
 
-    if sys.version_info[:3] < (3, 7, 4):
+    if sys.version_info[:3] < (3, 7, 4):  # pragma: no cover
         # Likewise for __copy__, except even Python 2 needs it.
         # See https://github.com/python/cpython/commit/3645d29a1dc2102fdb0f5f0c0129ff2295bcd768
         def __copy__(self):

src/persistent/tests/test_ring.py

@@ -27,9 +27,8 @@ def _next_oid(cls):
         cls.__next_oid += 1
         return cls.__next_oid
 
-    def __init__(self, oid=None):
-        if oid is None:
-            self._p_oid = self._next_oid()
+    def __init__(self):
+        self._p_oid = self._next_oid()
 
     def __repr__(self): # pragma: no cover
         return "<Dummy %r at 0x%x>" % (self._p_oid, id(self))