Chewy will join the S&P MidCap 400 on Wednesday.Shares of Chewy (CHWY 8.52%), the leading online pet products seller, were gaining today after the company effectively got a vote of confidence from S&P Global, as it said after hours on Friday that Chewy would join the S&P MidCap 400.As a result, Chewy stock was up 8.3% as of 12:15 p.m. ET. Chewy earns a stamp of approvalS&P Global, which manages indexes including the S&P MidCap 400, said that Chewy would replace Stericycle on the mid-cap index, as Stericycle ...