RFC 1034 defines absolute domain names as ending with dot:

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When a user needs to type a domain name, the length of each label is omitted and the labels are separated by dots (".").  Since a complete domain name ends with the root label, this leads to a printed form which ends in a dot.  We use this property to distinguish between:

   - a character string which represents a complete domain name
     (often called "absolute").  For example, "poneria.ISI.EDU."

   - a character string that represents the starting labels of a
     domain name which is incomplete, and should be completed by
     local software using knowledge of the local domain (often
     called "relative").  For example, "poneria" used in the
     ISI.EDU domain.
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I'll admit that it isn't common to specify absolute domain names, and many resolvers treat a domain name with an internal dot, but no terminal dot, as an absolute name.

I doubt in practice there are any email addresses that have a TLD name. 

There's some bpo issue where this was discussed in reference to the ipaddress module. I think the issues was canonicalizing names, and it was decided not to add trailing dot to make them absolute. I realize that logic doesn't directly apply here.

In spite of "com." being a valid domain name, I think it's reasonable to reject it as the domain part of an email address. But there should be a comment in the code as such.