After reading further, it seems that tls1.3 configuration will require hooks into
the SSL_CTX_set_ciphersuites() or SSL_set_ciphersuites() functions to configure
TLSv1.3 ciphersuites.  The functions SSL_CTX_get_ciphers() and SSL_get_ciphers()
will return the full list of ciphersuites that have been configured for both
TLSv1.2 and below and TLSv1.3.

The code in tls.c uses SSL_CTX_get_ciphers() and there is no function for the
ciphersuites functions.  As a consequence, it is not possible to configure tls1.3
for use with this version of tcltls.