Previously, mpycert.der was the Intermediate certificate which is regularly
re-issued by Letsencrypt.
Also changes ssl_cert.py to load the cert data from the same file as
test_sslcontext_client.py, so the DER string doesn't have to be pasted
into the source.
This work was funded through GitHub Sponsors.
Signed-off-by: Angus Gratton <angus@redyak.com.au>