Implement experimental support for public key pinning

In CPython 3.10 there were private APIs `SSLObject.get_verified_chain()` and `_ssl.Certificate.public_bytes(_ssl.ENCODING_DER)` methods added.

Ref: https://github.com/python/cpython/pull/25467

```python
import hashlib
import _ssl
fingerprints = [hashlib.sha256(cert.public_bytes(_ssl.ENCODING_DER)).hexdigest() for cert in sock._sslobj.get_verified_chain()]
```