remove python 3.7 support; changelog updates; use pip for readthedocs

[timmysilv]

this PR does 3 things:

• remove 3.7 support
• add changelog entries
• use pip instead of setuptools for the ReadTheDocs build. it fails tragically when installing the latest ProjectQ (>=0.8.0) with setuptools, ReadTheDocs recommends using pip, and we use it in our other repos, so it makes sense.

note: there are no CI actions available for this repo, so I made the necessary changes manually. I will get those prepared before the 0.30 release

[codecov[bot]]

Codecov Report

Merging #81 (f94db68) into master (462b129) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #81   +/-   ##
=======================================
  Coverage   93.99%   93.99%           
=======================================
  Files           6        6           
  Lines         233      233           
=======================================
  Hits          219      219           
  Misses         14       14           

Impacted Files	Coverage Δ
pennylane_pq/devices.py	99.41% <ø> (ø)

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