Make minimum Python version explicit

[albertz]

We need some check assert sys.version_info[:2] >= (3, 7) or so.
Also with some documentation of what actually we use.

• We rely on deterministic insertion order of dict, which requires Python >=3.6 (see here).

• We use from __future__ import annotations which requires Python >=3.7 (doc).

• typing.Protocol requires Python >=3.8.

• Slash (/) in function argument list to mark positional-only arguments (doc) requires Python >=3.8.

Originally posted by @albertz in #36 (comment)

Related discussion on RETURNN side: rwth-i6/returnn#487 (which currently still supports much older Python versions, also Python 2).