Add compatibility for Python 3.10

[antalszava]

Context:

Python 3.10 has been released early October and soon it became available with conda too:
https://anaconda.org/conda-forge/python

Description of the Change:

• Adds the compatibility tag for Python 3.10;
• Adds tests to be run on Python 3.10. too.

Benefits:
Python 3.10 compatibility.

Possible Drawbacks:
N/A

Related GitHub Issues:
N/A

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

Merging #1964 (7c94af4) into master (475c5e6) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1964   +/-   ##
=======================================
  Coverage   98.81%   98.82%           
=======================================
  Files         226      226           
  Lines       17348    17399   +51     
=======================================
+ Hits        17143    17194   +51     
  Misses        205      205           

Impacted Files	Coverage Δ
pennylane/vqe/vqe.py	100.00% <0.00%> (ø)
pennylane/drawer/tape_mpl.py	100.00% <0.00%> (ø)
pennylane/transforms/control.py	100.00% <0.00%> (ø)
pennylane/ops/qubit/attributes.py	100.00% <0.00%> (ø)
pennylane/optimize/shot_adaptive.py	100.00% <0.00%> (ø)
pennylane/gradients/parameter_shift.py	100.00% <0.00%> (ø)
pennylane/interfaces/batch/__init__.py	100.00% <0.00%> (ø)
pennylane/drawer/representation_resolver.py	99.37% <0.00%> (+<0.01%)	⬆️
pennylane/_qubit_device.py	98.70% <0.00%> (+<0.01%)	⬆️
pennylane/queuing.py	98.83% <0.00%> (+0.01%)	⬆️
... and 5 more

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[albi3ro]

🎉 🚀
Seems like there's a PR coming with the code factor fixes.