Is there any way to use variables in multiline string in Python?

So I have this as part of a mail sending script:

try:
    content = ("""From: Fromname <fromemail>
    To: Toname <toemail>
    MIME-Version: 1.0
    Content-type: text/html
    Subject: test

    This is an e-mail message to be sent in HTML format

    <b>This is HTML message.</b>
    <h1>This is headline.</h1>
    """)

...

    mail.sendmail('from', 'to', content)

And I'd like to use different subjects each time (let's say it's the function argument).

I know there are several ways to do this.

However, I am also using ProbLog for some of my other scripts (a probabilistic programming language based in Prolog syntax). As far as I know, the only way to use ProbLog in Python is through strings, and if the string is broke in several parts; example = ("""string""", variable, """string2"""), as well as in the email example above, there's no way I can make it work.

I actually have a few more scripts where using variables in multiline strings could be useful, but you get the idea.

Is there any way to make this work? Thanks in advance!