Bump `PYTHON3_VERSION_MIN`

[sobolevn]

Feature

Right now mypy says:

» mypy mypy/stubgen.py --python-version=3.2
usage: mypy [-h] [-v] [-V] [more options; see below]
            [-m MODULE] [-p PACKAGE] [-c PROGRAM_TEXT] [files ...]
mypy: error: argument --python-version: Python 3.2 is not supported (must be 3.4 or higher)

But, it does not look correct, because typeshed does not support 3.4 (currently it supports 3.7+).

So, how mypy can check 3.4?

I think that we need to bump it:

1. 3.4 is long gone
2. We don't actually plan any features with this support in mind
3. typshed does not support it
4. Users that need it can stay on older versions

Source: https://github.com/python/mypy/blame/2ebd51e881490f4d20635cde92ef9e3edbbad68c/mypy/defaults.py#L15-L15C20

Pitch

Let's use 3.7 as the minimal supported version. (Ideas about it are welcome)

After we decide the version info to be minimal, I will send a PR with the change :)

[kkirsche]

I think this is a great suggestion. I think it would make sense to intend to support all current active versions (leeway for new releases) and the last end-of-life version of Python.

That allows mypy to move forward without carrying the baggage of legacy code.

That would align with the suggestion of 3.7 per:
https://devguide.python.org/versions/

[hauntsaninja]

Good spot! Yeah, I'd go with whatever the vendored version of typeshed supports, so 3.7 currently.