Added back package for python 3.7

[vallsv]

It's a try to put back Python 3.7 packages.

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

I do have some suggestions for making it better though...

For recipe:

• It looks like the 'silx' output doesn't have any tests.
• Recipes should usually depend on matplotlib-base as opposed to matplotlib so that runtime environments do not require large packages like qt.

[tschoonj]

Conda-forge has officially stopped building packages for 3.7.

Instead you should switch to using more recent versions of Python in your conda environments.

[vallsv]

@tschoonj it's not true, according to the PR of the drop you still can pinpoint python 3.7.

conda-forge/conda-forge-pinning-feedstock#2623

"If any conda-forge package maintainers specifically need to keep Python 3.7 support they can ask on gitter for help"

So, will i have to ask on gitter for help?
Else ill just feed another channel with this recipe, but i prefer not.