Shopify is returning an invalid response with the 429's as the value for the Retry-After as "2.0"
It should not include the decimal point. The ABNF is 1*DIGIT which only allows digits and not the decimal point. The term "decimal interfere" indicates it is a base 10 interger.
Per the HTTP spec:
This is causing an exception to be thrown in the Python urlib3 library.
Thanks for bringing this up, I'll raise the issue with our developers for further clarification.