upgrading tf to 2.7.0

[scarrazza]

Ok, they decided to release 2.7.0 in less than 1 day from 2.6.2.
This PR implements the required changes, however we are forced to remove python3.6 support.

[stavros11]

Thanks for the update, it works for me. Regarding py3.6, I guess that we can still support it for other backends but we should update qibo's CI to exclude tensorflow backends from the py3.6 tests, right?

[codecov]

Codecov Report

Merging #17 (13de5a9) into main (c17f1f4) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main       #17   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines          528       528           
=========================================
  Hits           528       528           

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

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