By THÉRÈSE MARGOLIS From sleek, monochrome ensembles from the ’80s to brightly patterned hippie grannies gowns from the ’70s to buttoned-up tailored suits from the ’60s, there was a definitive retro feel when the high-end international Retail Fashion Group presented its new 2020 spring and summer collections in a seven-brand trunk show at Mexico City’s upscale Four Seasons Hotel on