"[C]areful attention to the requirements of [Rule] 23 remains . . . indispensable" even in cases "alleging racial or ethnic discrimination." E. Tex. Motor Freight Sys., Inc. v. Rodriguez, 431 U.S. 395, 405, 97 S. Ct. 1891, 52 L. Ed. 2d 453 (1977). Thus, "a Title VII class action, like any other class action, may o