OK. I found the answer to my own question. Page 128 of Practical
Django Projects, 2nd Edition, by James Bennett gives the solution.
Write the following:
{{ input_text|markdown:"safe" }}
Note there must be no space between the colon and the quote, despite
what is in the book. Note also that this solution is not provided in
the documentation of the markup package, which simply says "read the
source code for more details". Sheesh!
Rodney