Release dashboard: skip pre-pre-release versions with no branch yet

[hugovk]

3.16 hasn't started development yet but https://buildbot.python.org/#/release_status shows it:

This will be because the 3.16 schedule PEP has been merged (python/peps#4829) and we now generate the release PEPs from a single-source-of-truth TOML in the PEPs repo: https://github.com/python/peps/blob/main/release_management/python-releases.toml

We also generate https://peps.python.org/api/release-cycle.json from this, which includes:

{
"3.16": {
"branch": "",
"pep": 826,
"status": "",
"first_release": "2027-10-06",
"end_of_life": "2032-10",
"release_manager": "Savannah Ostrowski"
},
"3.15": {
"branch": "main",
"pep": 790,
"status": "feature",
"first_release": "2026-10-01",
"end_of_life": "2031-10",
"release_manager": "Hugo van Kemenade"
},
...

And that JSON is used for the release dashboard.

Let's skip versions with no branch set.

Note: I've not tested this fix locally or otherwise.

[hugovk]

Tested locally: ✅

[encukou]

Would you be OK with just de-emphasizing it?
This is where I find the release schedule PEP links :)

[encukou]

See #671 :)

[hugovk]

Closing in favour of #671.