MNT: Relax formulaic constraint

Author: effigies

setup.cfg

@@ -26,7 +26,7 @@ install_requires =
     scipy
     nibabel >=2.1
     pandas >=0.23
-    formulaic ~=0.2.4
+    formulaic >=0.2.4, <0.4  # Tested on 0.2.4 and 0.3.2
     sqlalchemy <1.4.0.dev0
     bids-validator
     num2words