There are two potential problems: with the airlines and with immigration.
With regard to the airlines, if the captain of a flight feels you are not healthy to fly, they may unilaterally ask you to be removed from the flight. Pilots are in general not medical experts. I can only imagine that this would be a problem if you boarded the flight with your foot bleeding all over the place, causing a mess and exposing other passengers to infection. Bandage the wound cleanly and cover it with a sock and closed toed shoe so people can't see it. The captain will never know, let alone care.
Immigration authorities sometimes deny entry to people who are sick, either because they have communicable diseases or because they might be a drain on the country's health system. http://www.mfa.gov.rs/en/consular-affairs/entry-serbia/entering-serbia-requirements indicates that Serbia is only concerned about people coming from epidemic areas. You should be fine on this front. Again, I would keep it covered. They won't know and they won't care.
In summary, I see no reason why this wound would prevent you from traveling if you keep it clean, bandaged, and covered. Carrying a doctor's certificate couldn't hurt and may be helpful in the extremely remote chance that you encounter problems.