chore: Add holidays optional dependency

Author: sondrelg

pyproject.toml

@@ -32,6 +32,11 @@ attrs = ">=17.4"
 # Backports
 "backports.zoneinfo" = { version="^0.2.1", python="<3.9" }
 
+[tool.poetry.extras]
+# Holidays can be installed to enable holiday-adjustments
+# in minimum due-date validation.
+holidays = ['holidays']
+
 [tool.poetry.dev-dependencies]
 check-manifest = "*"
 hypothesis = "*"
@@ -45,6 +50,7 @@ coverage = [
     { extras = ["toml"], version = "^6", python = "3.10" },
     { version = "^5 || ^6", python = ">=3.6!=3.10" },
 ]
+holidays = "^0.13"
 
 [build-system]
 requires = ["poetry-core>=1.0.0"]