pin colander <= 1.8.3

With the recent release of Colander 2.0, we need the current colander-tools
version to pin colander 1.8.3 because of a `colander.Mapping._impl()`
signature change (it now takes 5 params instead of 4), as well as
a dropped support for python <= 3.6.

setup.py

@@ -26,7 +26,7 @@
     url="https://github.com/platformsh/colander-tools",
     packages=find_packages(),
     install_requires=[
-        "colander",
+        "colander < 2",
         "pytz",
         "netaddr",
         "six",