Consider using PEP 440 compatible version strings

[agners]

Description

Today Supervisor uses calver versions with leading zeros (e.g. 2026.03.0). While this aligns with other Software project such as Ubuntu, and is generally a more "computer native" date format, it isn't compatible with PEP 440. Furthermore Home Assistant Core also chose not to add the leading zero.

Let's consider updating our version scheme to be PEP 440 compatible and align with Home Assistant Core.

Additional context

No response

[agners]

At that point we should also decide how nightly/local builds are versioned, see #6692.

For easier transition let's plan to update this once we have two digits in the minor version part, so in October.