ci: fix build as sqlalchemy is not supported yet

[juarezr]

This PR has the objective of restricting the maximum supported version of SQLAlchemy.

Changes

1. Added in requirements-database.txt: SQLAlchemy>=1.3.6,<2.0
2. Added in setup.py: SQLAlchemy>=1.3.6,<2.0

For SQLAlchemy >=2.0 check #648.

Checklist

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• [ ] Source Code guidelines:
  • [ ] Includes unit tests
  • [ ] New functions have docstrings with examples that can be run with doctest
  • [ ] New functions are included in API docs
  • [ ] Docstrings include notes for any changes to API or behavior
  • [ ] All changes are documented in docs/changes.rst
• [x] Versioning and history tracking guidelines:
  • [x] Using atomic commits whenever possible
  • [x] Commits are reversible whenever possible
  • [ ] There are no incomplete changes in the pull request
  • [x] There is no accidental garbage added to the source code
• [X] Testing guidelines:
  • [X] Tested locally using tox / pytest
  • [X] Rebased to master branch and tested before sending the PR
  • [x] Automated testing passes (see CI)
  • [x] Unit test coverage has not decreased (see Coveralls)
• [X] State of these changes is:
  • [ ] Just a proof of concept
  • [ ] Work in progress / Further changes needed
  • [X] Ready to review
  • [X] Ready to merge