nix: relax pillow dep

Signed-off-by: Sefa Eyeoglu <[email protected]>

Author: Scrumplex

nix/packages/modmail.nix

@@ -115,6 +115,7 @@ buildPythonApplication {
     "attrs"
     "defusedxml"
     "packaging"
+    "pillow"
     "uvloop"
   ];