Use Windows certificate store in Python sample

[forderud]

Python truststore issue: https://github.com/sethmlarson/truststore/issues/78

The WebClient.py sample script currently relies on loading the client certificate (incl. private key) from a file instead of the Windows certificate store. This makes it incompatible with secure TPM-based storage of the private key.

Resources

It seems like Python 3.10 includes improvements in this area that will hopefully enable direct usage of the Windows certificate store (see https://sethmlarson.dev/blog/experimental-python-3.10-apis-and-trust-stores).

Python SSL sources:

• https://github.com/python/cpython/blob/main/Lib/ssl.py (_load_windows_store_certs("My", ssl.Purpose.CLIENT_AUTH) looks interesting but doesn't seem to help)
• Underlying ssl_collect_certificates: https://github.com/python/cpython/blob/main/Modules/_ssl.c#L5380

OpenSSL sources:

• OpenSSL CAPI back-end: https://github.com/openssl/openssl/blob/master/engines/e_capi.c#L1413 (seem to be undocumented)