Fix cibuildwheel for Python 3.8

[chaeyeunpark]

Before submitting

Please complete the following checklist when submitting a PR:

• All new features must include a unit test.
  If you've fixed a bug or added code that should be tested, add a test to the
  tests directory!

• All new functions and code must be clearly commented and documented.
  If you do make documentation changes, make sure that the docs build and
  render correctly by running make docs.

• Ensure that the test suite passes, by running make test.

• Add a new entry to the .github/CHANGELOG.md file, summarizing the
  change, and including a link back to the PR.

• Ensure that code is properly formatted by running make format.

When all the above are checked, delete everything above the dashed
line and fill in the pull request template.

---

Context: Fix issue #210

Description of the Change:

• Add ENABLE_WARNINGS=OFF for building wheels
• Github workflow for cibuildwheel correctly use cl.exe (thus discarding MinGW dependencies).
• Loading lightning_qubit_ops is rollbacked.

Benefits:

Possible Drawbacks:

Related GitHub Issues:

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit .github/CHANGELOG.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 #211 (ecf1f2f) into master (7515848) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #211      +/-   ##
==========================================
- Coverage   99.64%   99.63%   -0.01%     
==========================================
  Files           4        4              
  Lines         278      277       -1     
==========================================
- Hits          277      276       -1     
  Misses          1        1              

Impacted Files	Coverage Δ
pennylane_lightning/lightning_qubit.py	99.48% <100.00%> (-0.01%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7515848...ecf1f2f. Read the comment docs.

[github-actions]

Test Report (C++) on Ubuntu

       1 files  ±0         1 suites  ±0   0s ⏱️ ±0s
   388 tests ±0     388 ✔️ ±0  0 💤 ±0  0 ❌ ±0 
2 247 runs  ±0  2 247 ✔️ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit ecf1f2f. ± Comparison against base commit 7515848.

♻️ This comment has been updated with latest results.

[mlxd]

I am guessing the tiny drop in coverage is from here. Maybe adding a quick test to check for the expected exception types may fix the drop, and all the other tests to continue/pass. What do you think?

[chaeyeunpark]

Hi Lee, as this became a part of #202, I will close the PR. I was using this PR as a test purpose 🙃

[chaeyeunpark]

As PR #202 now contains this PR, we close This PR. #202 will also close issue #211