Support for python 3.8

[thisac]

Context:
Adds support for Python 3.8.

Description of the Change:
Removes cp38-* from the environment variable CIBW_SKIP in .circleci/config.yml (there was no cp38-* in appveyor.yml), and updates the README.rst and setup.py files.

Benefits:
Support for Python 3.8, which is good, especially since it's becoming the default Python version for many distributions.

Possible Drawbacks:
N/A

Related GitHub Issues:
closes #133

[codecov]

Codecov Report

Merging #138 into master will increase coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #138      +/-   ##
==========================================
+ Coverage   97.61%   97.74%   +0.12%     
==========================================
  Files          12       12              
  Lines         839      887      +48     
==========================================
+ Hits          819      867      +48     
  Misses         20       20

Impacted Files	Coverage Δ
thewalrus/fock_gradients.py	100% <ø> (ø)	⬆️
thewalrus/quantum.py	100% <100%> (ø)	⬆️

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[josh146]

Thanks @thisac! If you click on the logs for the osx-wheels, you can see that Python 3.8 is working fine, which is great. However, the linux-wheels and the appveyor builds seem to be stopping at Python 3.7 🤔 Is there more info over at the https://github.com/joerick/cibuildwheel repo to help explain how we activate 3.8 for Linux/Appveyor?

[josh146]

(Perhaps CircleCI and Appveyor are installing an old version of cibuildwheel that doesn't have 3.8 support yet, so the pip command needs to install the latest version?

[josh146]

Seems to be working on appveyor, see https://ci.appveyor.com/project/josh146/hafnian/builds/31112782#L1592. Awesome!

[josh146]

Everything seems to have worked! The last part of this PR will be:

• Updating the classifiers in the setup.py file, so that PyPI knows that The Walrus supports 3.8

• Updating the table in the readme, to indicate 3.8 support. In fact, I would probably delete the table, and instead just have a bullet point list (the table is hard to maintain).

[thisac]

Everything seems to have worked! The last part of this PR will be:

* Updating the `classifiers` in the `setup.py` file, so that PyPI knows that The Walrus supports 3.8

* Updating the table in the readme, to indicate 3.8 support. In fact, I would probably delete the table, and instead just have a bullet point list (the table is hard to maintain).

The classifiers in setup.py are already updated.

I'll fix the table right away. 🙂