Vivarium SyntaxWarning on Python 3.12

[aflaxman]

When testing Pseudopeople with Python 3.12, I came across a warning worth addressing.

When importing pseudopeople on Python 3.12, I see:

.../vivarium/framework/values/post_processors.py:60: SyntaxWarning: invalid escape sequence '\p'

I believe that this is a bug in vivarium (not pseudopeople). Python 3.12 became stricter about invalid
escape sequences in strings. The fix needs to happen in vivarium - the docstring at line 60
of post_processors.py should use a raw string r"""...""" or escape as \\p.