gh-118658: Modify cert generation script to extract cert3.pem

[felixfontein]

An alternative to #124598 that's simpler. The additional data in cert3.pem shouldn't affect the tests.

Edit: doesn't work since PEM_cert_to_DER_cert doesn't like PEM comments.

• Issue: Inconsistent return types between SSLSocket and SSLObject certificate chain APIs #118658

[felixfontein]

The additional data in cert3.pem shouldn't affect the tests.

Apparently it does:

ERROR: test_certificate_chain (test.test_ssl.TestPostHandshakeAuth.test_certificate_chain) ---------------------------------------------------------------------- Traceback (most recent call last): File "/home/runner/work/cpython/cpython/Lib/test/test_ssl.py", line 4707, in test_certificate_chain expected_ee_cert = ssl.PEM_cert_to_DER_cert(f.read()) File "/home/runner/work/cpython/cpython/Lib/ssl.py", line 1498, in PEM_cert_to_DER_cert raise ValueError("Invalid PEM encoding; must start with %s" % PEM_HEADER) ValueError: Invalid PEM encoding; must start with -----BEGIN CERTIFICATE----- 

So PEM_cert_to_DER_cert doesn't like PEM with comments. I guess in that case #124598 is preferable.