bpo-31734: Raise a ValueError instead of crashing, when the get() method of an uninitialized sqlite3.Cache object is called

[orenmn]

In addition:

• Prevent a Cache object from becoming partially initialized in case its __init__() method failed after it was already initialized. (And thus prevent the case of raising a SystemError by the get() method of a partially initialized Cache object. The fix for that would also prevent refleaks in case of calling the __init__() method of a Cache object more than once.)
• Prevent a memory leak in the deallocation of an uninitialized Cache object.
• Add tests to test_sqlite, to verify that the crash and the SystemError are no more.

https://bugs.python.org/issue31734

[berkerpeksag]

Closing as per https://bugs.python.org/issue31734#msg322171