Feature: implement python version substitution in conf.py

[psobolewskiPhD]

Description

This PR is a leftover from #51
The idea was from @Czaki Chris-N-K#1 (comment)
Basically, the python version shown in various places should be globally set in conf.py once and then substitutions used.

At the moment, I've set the python version to 3.9, but I at some point we should consider switching to 3.10.

Type of change

• Fixes or improves existing content
• Adds new content page(s)
• Fixes or improves workflow, documentation build or deployment

References

Final checklist:

• My PR is the minimum possible work for the desired functionality
• I have commented my code, particularly in hard-to-understand areas
• I have added alt text to new images included in this PR

[psobolewskiPhD]

@Czaki I added python_minimum_version and python_version_range as substitutions and I think I've now got the substitutions used in all the places the python version was mentioned in docs, outside of release notes and such.

[melissawm]

This looks good to me. I know this may be out of scope for this PR, but while we're at it, could we also bump the version mentioned in the quickstart guide? Currently it says

This tutorial uses napari 0.4.14. 

[Czaki]

I have found these places that for me should be addressed in this PR:

docs/docs/tutorials/fundamentals/installation.md

Lines 215 to 216 in d1c2878

	name like `napari-0.3.7-macOS.zip`. After unzipping you will have a file with a
	name like `napari-0.3.7.dmg`. Double clicking the `dmg` will open a new finder

docs/docs/tutorials/fundamentals/installation.md

Lines 256 to 258 in d1c2878

	name like `napari-0.3.7-Windows.zip`. Unzip the bundle (you may like to use a
	tool like [7-zip](https://www.7-zip.org/) for this) and double click on msi
	file, eg: `napari-0.3.7.msi`

docs/docs/tutorials/fundamentals/quick_start.md

Line 27 in d1c2878

This tutorial uses napari 0.4.14. <br>

docs/docs/tutorials/fundamentals/quick_start.md

Lines 53 to 55 in d1c2878

	[Linux installation](https://github.com/napari/napari/releases/download/v0.4.16/napari-0.4.16rc7-Linux-x86_64.zip)<br>
	[macOS installation](https://github.com/napari/napari/releases/download/v0.4.16/napari-0.4.16rc7-macOS-x86_64.zip)<br>
	[Windows installation](https://github.com/napari/napari/releases/download/v0.4.16/napari-0.4.16rc7-Windows-x86_64.zip)<br>

[psobolewskiPhD]

@Czaki I saw those—are any of them still actual?
I think the whole bundle is in flux to the conda bundle?
🤷‍♂️

[Czaki]

The content of installation.md is connected with artifacts from https://github.com/napari/napari/blob/1eb5e3142063630a49dd30705c9aa2ff3cc1866b/.github/workflows/make_bundle.yml - it is pure napari repository.

We still have workflow for conda build https://github.com/napari/napari/blob/1eb5e3142063630a49dd30705c9aa2ff3cc1866b/.github/workflows/make_bundle_conda.yml

So when looking at the current state of the repository, both are actual. And I do not see an Issue or PR for removing this now.

[psobolewskiPhD]

This looks good to me. I know this may be out of scope for this PR, but while we're at it, could we also bump the version mentioned in the quickstart guide? Currently it says

This tutorial uses napari 0.4.14. 

@melissawm I didn't change that because I'm not sure we want versions in guides/tutorials changing automatically when the variable is bumped in conf.py. It's probably a place where it's best to manually bump the number after ensuring the content matches the new napari version. Of course, in this case I doubt much changed between 0.4.14 and 0.4.17...

[psobolewskiPhD]

@Czaki I updated the quick_start links to use the substituted version to link directly to latest stable. However, looking at it more, I think the whole guide for the bundle app
https://napari.org/stable/tutorials/fundamentals/installation.html#install-as-a-bundled-app
should probably be updated—see #67
Looks like @jaimergp has a WIP PR for this #74

So I think this is out-of-scope of this PR, which is intended to provide the framework of substitutions.

I did go ahead and add a link in the first paragraph to go directly to the current stable release using napari_version.