Use the system certificate store instead of certifi

I'm trying to run "az upgrade", which fails beautifully presumably due to the corporate proxy we have. https://github.com/Azure/azure-cli/issues/17938#issuecomment-890038445 gives some information on how to solve it, including a link to https://docs.microsoft.com/en-us/cli/azure/use-cli-effectively#work-behind-a-proxy.

The outlined mechanism seems ... convoluted. It suggests that I should edit `C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\Lib\site-packages\certifi\cacert.pem`. This requires that I acquire the corporate MITM certificate (it's probably easy, but I don't know how), and that I'm comfortable with the file format. Also, every user would have to do this on every computer.

* Would it be possible to bypass all of this, using the system certificate store instead?

```
> az upgrade
This command is in preview and under development. Reference and support levels: https://aka.ms/CLI_refstatus
You already have the latest azure-cli version: 2.27.1
Upgrading extensions
Checking update for azure-devops
An error occurred whilst updating.
Please ensure you have network connection. Error detail: HTTPSConnectionPool(host='github.com', port=443): Max retries exceeded with url: /Azure/azure-devops-cli-extension/releases/download/20210805.1/azure_devops-0.20.0-py2.py3-none-any.whl (Caused by SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate (_ssl.c:1125)')))
Failed to update. Rolled azure-devops back to 0.18.0.
Checking update for resource-graph
No updates available for 'resource-graph'. Use --debug for more information.
Upgrade finished.
```

## Environment Summary
```
Windows-10-10.0.18362-SP0
Python 3.8.9
Installer: MSI

azure-cli 2.27.1

Extensions:
azure-devops 0.18.0
resource-graph 2.1.0
```