Bump HepMC3 to 3.2.7

[andriish]

Have you:

• [x] Run brew audit --strict --online <formula> (where <formula> is the name of the formula you're submitting) and corrected all errors?
• [x] Built your formula locally prior to submission with brew install <formula>?

Updates to existing formula: have you

• [x] Checked that the tests still pass?