Fix upgrade process, stretch support and linter

[Jibec]

Problems

• there's a strange bug with python in https://github.com/YunoHost-Apps/searx_ynh/pull/33
• the package doesn't build in stretch, because of pybabel
• restore was broken because of missing dependencies
• the linter fails for tiny details

Solution

• prevent pip to try to use system wide packages
  • this enables to remove the pybabel package
    • fix dependencies

PR Status

Could be reviewed and tested. It worked in x86 both package_check and VM. Needed: arm tests.

Validation

---

Minor decision

• Upgrade previous version :
• [ ] Code review :
• [ ] Approval (LGTM) :
• [ ] Approval (LGTM) :
  When the PR is mark as ready to merge, you have to wait for 3 days before really merge it.

[maniackcrudelis]

Could you, please, rework this PR to start from the branch "add_ynh_check_starting", to avoid all this conflict and have a clear view about what's new in this PR.

And also explain what did you do.

[maniackcrudelis]

This PR does not seem to fix the issue in https://github.com/YunoHost-Apps/searx_ynh/pull/33 Can we have any explanation about this PR @Jibec ?

[maniackcrudelis]

And as I already said, you need to restart this PR from the correct branch. So please, close this PR, and redo the needed commits from the correct branch.

Also, this PR does not fix the issue in #33. So again, did you reproduce the error ?

I think that the fixes for the linter and missing packages should be done in another PR, in order to be merged separately. Since it seems legit.

PS: Avoid branch name like "origin/add_ynh_check_starting" which is difficult to handle for git cli and for our CI.