The future Mrs. Paul McCartney was more concerned with buses than wedding bells at Tuesday’s MTA board meeting in Manhattan. Nancy Shevell demurred when asked about their upcoming nuptials, saying, “I don’t want to discuss that at the MTA.” But the chic brunette — the chair of the transit agency’s bus committee — did brighten up when we congratulated her on her impending wedding. “Thank you!” Shevell told The Post’s Jennifer Fermino. “I appreciate that. Thank you.”