Description of the issue/feature this PR addresses:
There is no deepcopy support for an Odoo model instance.

Current behavior before PR:
```
>>> deepcopy(self.env.user)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/usr/lib/python3.6/copy.py", line 180, in deepcopy
    y = _reconstruct(x, memo, *rv)
  File "/usr/lib/python3.6/copy.py", line 280, in _reconstruct
    state = deepcopy(state, memo)
  File "/usr/lib/python3.6/copy.py", line 150, in deepcopy
    y = copier(x, memo)
  File "/usr/lib/python3.6/copy.py", line 240, in _deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "/usr/lib/python3.6/copy.py", line 180, in deepcopy
    y = _reconstruct(x, memo, *rv)
  File "/usr/lib/python3.6/copy.py", line 274, in _reconstruct
    y = func(*args)
  File "/usr/lib/python3.6/copyreg.py", line 88, in __newobj__
    return cls.__new__(cls, *args)
TypeError: __new__() missing 3 required positional arguments: 'cr', 'uid', and 'context'
```

Desired behavior after PR is merged:
```
>>> deepcopy(self.env.user)
>>> res.users[1]
```



--
I confirm I have signed the CLA and read the PR guidelines at www.odoo.com/submit-pr