Fix fixtures for Python3.7

[CasperWA]

Description:

Fix issue with fancy pytest fixtures for running the tools as a Python module for Python 3.7.

Basically, the fixtures have been reimplemented by splitting the single tool fixture into two separate fixtures:

• get_tool: Import a tool as a module.
  Essentially, one uses this fixture in a test and then runs ontodoc = get_tool("ontodoc") to import the tools/ontodoc file as a module. It can now be tested by running its main() function.
• rename_tools: Add a .py extension to all tools.
  This fixture runs prior to all tests in this module (the tests/tools/ folder). First, it adds a .py suffix to all the tool file names to make them importable as a module. Then it stops executing for a while (yields) and then all the tests can be run.
  Then after they're done it reverts the file names, by removing the .py suffix. Furthermore, it returns sys.path to its original state prior to running the tests; to make the imports of the tools work, the tools folder had to be added to the sys.path.

I updated the doc-strings for all the tests to be more clear about where the fixtures are coming from as well, and tried to minimize the imports outside the test functions as much as possible.

Type of change:

• Bug fix.
• New feature.
• Documentation update.

Checklist:

This checklist can be used as a help for the reviewer.

• Is the code easy to read and understand?
• Are comments for humans to read, not computers to disregard?
• Does a new feature has an accompanying new test (in the CI or unit testing schemes)?
• Has the documentation been updated as necessary?
• Does this close the issue?
• Is the change limited to the issue?
• Are errors handled for all outcomes?
• Does the new feature provide new restrictions on dependencies, and if so is this documented?

Comments:

[francescalb]

Excellent!