Issue 31770: crash and refleaks when calling sqlite3.Cursor.__init__() more than once

The following code crashes:
import sqlite3
import weakref
def callback(*args):
    pass

connection = sqlite3.connect(":memory:")
cursor = sqlite3.Cursor(connection)
ref = weakref.ref(cursor, callback)
cursor.__init__(connection)
del cursor
del ref

IIUC, this is because pysqlite_cursor_init() (in Modules/_sqlite/cursor.c) sets
`self->in_weakreflist` to NULL, and thus corrupts the weakref list. Later,
clear_weakref() (in Objects/weakrefobject.c) tries to remove a reference from
the corrupted list, and crashes.

In every other place (that i saw) where such a weakreflist field is used, it is
set to NULL right after allocating the object (usually in __new__()), or just
not set at all, e.g. in `functools.partial`.
So since PyType_GenericNew() is the __new__() of sqlite3.Cursor, ISTM that the
simplest solution is to not touch `self->in_weakreflist` at all in
pysqlite_cursor_init().


Also, the following code results in refleaks:
import sys
import sqlite3
connection = sqlite3.connect(":memory:")
cursor = sqlite3.Cursor(connection)
refcount_before = sys.gettotalrefcount()
cursor.__init__(connection)
print(sys.gettotalrefcount() - refcount_before)  # should be close to 0

This is because pysqlite_cursor_init() doesn't decref before assigning to
fields of `self`.


I would open a PR to fix this soon.

New changeset e56ab746a965277ffcc4396d8a0902b6e072d049 by Victor Stinner (Oren Milman) in branch 'master':
bpo-31770: Prevent a crash and refleaks when calling sqlite3.Cursor.__init__() more than once (#3968)
https://github.com/python/cpython/commit/e56ab746a965277ffcc4396d8a0902b6e072d049

New changeset 4b544aadd54fd2337d2ab5c137389cae8d1fd781 by Victor Stinner (Miss Islington (bot)) in branch '2.7':
bpo-31770: Prevent a crash and refleaks when calling sqlite3.Cursor.__init__() more than once (GH-3968) (#4302)
https://github.com/python/cpython/commit/4b544aadd54fd2337d2ab5c137389cae8d1fd781

New changeset 9684cf69e32ae442c7be54521073ac78557f3bbf by Victor Stinner (Miss Islington (bot)) in branch '3.6':
bpo-31770: Prevent a crash and refleaks when calling sqlite3.Cursor.__init__() more than once (GH-3968) (#4301)
https://github.com/python/cpython/commit/9684cf69e32ae442c7be54521073ac78557f3bbf

Thank you Oren Milman for your bug report and the bug fix! I merged your PR and backported it Python 3.6 and 2.7.