SSLContext.load_verify_locations(cadata=<DER>) fails with [ASN1: NOT_ENOUGH_DATA] when linked against OpenSSL 3.0.21+

Bug report

Bug description:

When CPython is linked against OpenSSL 3.0.21 (and likely any release carrying the CVE-2026-34180 ASN.1 hardening), SSLContext.load_verify_locations(cadata=) raises:

ssl.SSLError: [ASN1: NOT_ENOUGH_DATA] not enough data (_ssl.c:NNNN)

even for a single, perfectly valid DER certificate. On Windows this breaks ssl.create_default_context() outright, because SSLContext._load_windows_store_certs() loads the system trust store via cadata=. The practical effect is that all default-context HTTPS fails (e.g. importing aiohttp, which builds a default context at import time, crashes).

Reproducer (platform-independent)

import ssl
pem = ssl.get_server_certificate(("www.python.org", 443))
der = ssl.PEM_cert_to_DER_cert(pem)

ctx = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
ctx.load_verify_locations(cadata=der)   # raises [ASN1: NOT_ENOUGH_DATA] on OpenSSL 3.0.21+
print("loaded OK")

OpenSSL 3.0.15 (and earlier): prints loaded OK.
OpenSSL 3.0.21: raises ssl.SSLError: [ASN1: NOT_ENOUGH_DATA].

The certificate itself is fine — calling d2i_X509 directly against the same libcrypto-3.dll parses it successfully; only the path through _ssl fails.

Root cause
In Modules/_ssl.c, _add_ca_certs() reads certs in a loop until d2i_X509_bio() returns NULL, then inspects the leftover error to distinguish a clean end-of-buffer from a real failure. It only accepts ASN1_R_HEADER_TOO_LONG as the benign EOF marker:

} else if ((filetype == SSL_FILETYPE_ASN1) &&
                (ERR_GET_LIB(err) == ERR_LIB_ASN1) &&
                (ERR_GET_REASON(err) == ASN1_R_HEADER_TOO_LONG)) {
    /* EOF ASN1 file, not an error */
    ERR_clear_error();
    retval = 0;
}

OpenSSL 3.0.21 now reports end-of-buffer as ASN1_R_NOT_ENOUGH_DATA (reason 142) instead of ASN1_R_HEADER_TOO_LONG. That reason isn't whitelisted, so the function falls through to the generic error branch and raises, discarding the load. Confirmed by reading ERR_peek_last_error() after the final d2i_X509_bio() on a single-cert buffer: 0x0680008E = asn1 ... not enough data.

Proposed fix
Treat ASN1_R_NOT_ENOUGH_DATA as a clean EOF alongside ASN1_R_HEADER_TOO_LONG:

} else if ((filetype == SSL_FILETYPE_ASN1) &&
                (ERR_GET_LIB(err) == ERR_LIB_ASN1) &&
                ((ERR_GET_REASON(err) == ASN1_R_HEADER_TOO_LONG) ||
                 (ERR_GET_REASON(err) == ASN1_R_NOT_ENOUGH_DATA))) {
    /* EOF ASN1 buffer, not an error (OpenSSL 3.0.21+ reports the
       end-of-data condition as ASN1_R_NOT_ENOUGH_DATA). */
    ERR_clear_error();
    retval = 0;
}

With this change, load_verify_locations(cadata=...) succeeds, the Windows store loads fully, and TLS verification works again.

Environment
CPython 3.11.15 (self-built, Windows x64, MSVC), linked against self-compiled OpenSSL 3.0.21.
Windows 11.
Also expected to affect any build/version of CPython linked against OpenSSL ≥ 3.0.21 (or whichever release introduced the reason-code change), on any platform, for the DER cadata path.

CPython versions tested on:

3.11

Operating systems tested on:

Windows

Start in Modules/_ssl.c at _add_ca_certs(), especially the d2i_X509_bio() loop and its ERR_peek_last_error() handling. Reproduce the DER cadata failure with the provided ssl example, then verify that the single certificate loads successfully and that Windows default-context trust-store loading works with the updated EOF handling.