Want to get some post-Thanksgiving exercise and help a good cause at the same time? Lakota YouthStay is teaming up with the Medford fitness center Reimagym to present a 45-minute fitness class as a fundraiser for cultural exchange program that brings Native American youth to Medford each summer. Participants may attend the 45-minute session in-person at Reimagym — a gym that caters to people who have shied away from traditional fitness spaces — or virtually. The class is free, but…