Update sha256 hash of pluggy.

Refs https://github.com/pytest-dev/pluggy/issues/134 where we got the
official confirmation of the maintainer.

The hashes changed but the actual code didn't so nothing risky happened.

requirements/dev.txt

@@ -115,7 +115,7 @@ attrs==17.4.0 \
     --hash=sha256:a17a9573a6f475c99b551c0e0a812707ddda1ec9653bed04c13841404ed6f450 \
     --hash=sha256:1c7960ccfd6a005cd9f7ba884e6316b5e430a3f1a6c37c5f87d8b43f83b54ec9
 pluggy==0.6.0 \
-    --hash=sha256:7f8ae7f5bdf75671a718d2daf0a64b7885f74510bcd98b1a0bb420eb9a9d0cff
+    --hash=sha256:d345c8fe681115900d6da8d048ba67c25df42973bda370783cd58826442dcd7c
 isort==4.3.4 \
     --hash=sha256:ec9ef8f4a9bc6f71eec99e1806bfa2de401650d996c59330782b89a5555c1497 \
     --hash=sha256:1153601da39a25b14ddc54955dbbacbb6b2d19135386699e2ad58517953b34af \