The function link is here: https://github.com/python/cpython/blob/e42b705188271da108de42b55d9344642170aa2b/Lib/http/cookiejar.py#L521

The function returned true for "foo!bar.com" which is false positive and the test case is taken from the list of invalid domain names present in Guava test suite https://github.com/google/guava/blob/581ba1436ebaa54a7f5d0f1db8cc4da0ca72127e/android/guava-tests/test/com/google/common/net/InternetDomainNameTest.java#L81

Domain names with dots at the end are fully qualified domain names. (reference:
1.https://stackoverflow.com/questions/19480767/domain-names-with-dots-at-the-end#answer-19498025
2. https://en.wikipedia.org/wiki/Fully_qualified_domain_name#Syntax
)
the function returned false for "woo.com." which is a false negative.
The test case is again taken from list of valid domain names provided by guava test suite (https://github.com/google/guava/blob/581ba1436ebaa54a7f5d0f1db8cc4da0ca72127e/android/guava-tests/test/com/google/common/net/InternetDomainNameTest.java#L65)