Fix backend pickle

[stavros11]

The following:

import qibo
import dill

qibo.set_backend("qibolab")
dill.dump_session("session.pkl")
dill.load_session("session.pkl")
qibo.set_backend("numpy")

namely changing the backend after loading a session from pickle fails with AttributeError. Here the __getitem__ and __setitem__ methods are update to pickle and load all the attributes of the Backend object to fix this issue.

[codecov]

Codecov Report

Merging #581 (b1a6110) into master (2deee67) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #581   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           86        86           
  Lines        12981     12989    +8     
=========================================
+ Hits         12981     12989    +8     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/qibo/backends/__init__.py	100.00% <100.00%> (ø)
src/qibo/tests/test_backends_init.py	100.00% <100.00%> (ø)

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