cidict no longer defines a .copy() method

Issue description:

At some point, cidict.copy() disappeared.

Steps to reproduce: 

```python3
from ldap.cidict import cidict
d = cidict()
d.copy()
```

Used to work, now `.copy()` is no longer a method.

Operating system: Linux
Python version: 3.8
python-ldap version: 3.3.1