Not just any Python 3.x version is supported - update readme

[Noiredd]

Description of the change

As shown in #2083, Python 3.4 does not support the syntax used in this project. Therefore, I propose to update the readme file to mention that explicitly. Fixing the code to make it work in 3.4 is way too much pointless work, but a simple readme change might save some users (like myself) from the trouble of trying to make it work.
Due to the trivial nature of this change, I took the liberty of skipping some sections of the PR template.

Minimal example of the bug

Execute the following shell command:
python3.4 -c "import hyperspy.api"

[thomasaarholt]

I can supply a fix in the future if someone has a need for hyperspy in Python 3.4.

[Noiredd]

@thomasaarholt Is it worth the effort? Not sure how many people use old versions - I will be updating to 3.5 or maybe some later version myself and give it a try then.

[thomasaarholt]

In that case I'll wait until someone requests support for it. It won't take long to do, but it'll make the code a bit longer.

[ericpre]

As mentioned in #2083, the main problem is not to fix it but to maintain it. Actually, after a quick look at this hyperspy 1.3 should work with python 3.4. @Noiredd, could you please confirm that 1.3 is working fine?

[ericpre]

It would be nice to add a badge for the supported python version. Not sure how it works but one way may be get it through pypi and the classifiers of the setup.py file, which may need to be updated.

[ericpre]

Once we know what version of hyperspy can support 3.4, mention it here.

[thomasaarholt]

I wasn't intending to use f-strings in #1598, but @ericpre your comment in that PR re-raises this valid issue - what version of python do we intend to support in future releases?

Currently, it looks like we fully support python 3.5, but there are features like f-strings and data classes that require versions 3.6 and 3.7 respectively.

Do you and @francisco-dlp have any strong feelings about this?

[francisco-dlp]

I very much like f-strings because I think that it makes the code more compact and readable. Should we drop support for 3.5 to be able to start using them? I don't know. From our point of view, certainly yes, since supporting just the latest 2 Python releases decreases the maintenance burden. However, there is a chance that there are users out there who cannot upgrade the Python version for whatever reason. What about opening an issue about it to see what users think about it?

[thomasaarholt]

We could consider emailing out to the users to see if any are stuck with certain versions of python. There will be many users who aren't on Github.

[jat255]

This is similar to the decision to no longer support Python 2. I don't think it is at all unreasonable to say that if you want an updated version of HyperSpy, you must be using a version of Python released in the past 3 (or 4, 5, n) years. Some users may be excluded by this, but at the same time we have very limited development resources, and I think those should be focused on making use of the latest tools available.

…

On Thu, May 16, 2019 at 11:13 AM Thomas Aarholt ***@***.***> wrote: We could consider emailing out to the users to see if any are stuck with certain versions of python. There will be many users who aren't on Github. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#2086?email_source=notifications&email_token=AAJYCXJ3MSIZUAAMU5QMVZLPVV22RA5CNFSM4GAOTOU2YY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGODVSEE2Y#issuecomment-493109867>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAJYCXPYP7Q5Z7WC23BARWLPVV22RANCNFSM4GAOTOUQ> .

-- Joshua Taillon www.joshuataillon.com

[ericpre]

Fixed in #2212.