table.copy should return an instance of table

[Julian]

table.copy appears to return an instance of dict, not table:

⊙  rm -rf venv; python3.7 -m venv venv; venv/bin/python -m pip install --quiet tomlkit; venv/bin/python -m pip list | grep tomlkit; venv/bin/python -c '                julian@Air
import tomlkit
table = tomlkit.table()
print(type(table.copy()))'
/Users/julian/Desktop/venv/lib/python3.7/site-packages/pip/_vendor/msgpack/fallback.py:133: DeprecationWarning: encoding is deprecated, Use raw=False instead.
  unpacker = Unpacker(None, max_buffer_size=len(packed), **kwargs)
tomlkit    0.5.8
<class 'dict'>

Besides having the type change during copying, this makes doing immutable changes to tables more difficult (copying a table and mutating the copy).