Is there official guide for Python 3.x release lifecycle?

Yes, you could look at the table in the Pythons Developer Guide for most releases. Specifically Python 3.3 will have security fixes until 2017-09-29.

Additionally, appropriate PEPs exist (google-able or from the devguide table) for each branch where a lifespan section specifies these. For 3.3 in PEP 398:

3.3 will receive bugfix updates approximately every 4-6 months for approximately 18 months. After the release of 3.4.0 final, a final 3.3 bugfix update will be released. After that, security updates (source only) will be released until 5 years after the release of 3.3 final, which will be September 2017.

For the rest of the actively maintained Pythons, the PEPs are:

• Python 3.9 PEP 596
• Python 3.8 PEP 569
• Python 3.7 PEP 537
• Python 3.6 PEP 494
• Python 3.5 PEP 478

See the Lifespan and Schedule sections for details on these.

As of this answer (January 2021):

• Python 3.4: reached End Of Life
• Python 3.5: reached End Of Life
• Python 3.6: security fixes only, nobug fixes will be provided; End Of Life: 2021-12-23
• Python 3.7: security fixes only, nobug fixes will be provided; End Of Life: 2023-06-27
• Python 3.8: security fixes AND bug fixes will be provided; End Of Life: 2024-10
• Python 3.9: security fixes AND bug fixes will be provided; End Of Life: TBD

Source: https://devguide.python.org/#status-of-python-branches

Also notice that Python release cycle has changed to 12 months since Python 3.9. See PEP-602 for more details. It used to be 18 months for Python 3.8 and prior releases. This means we should see Python 3.9 before end of year 2020!

Each Python release gets a Release Schedule PEP which includes information about the planned lifespan. For 3.3: https://www.python.org/dev/peps/pep-0398/#id11