isOutputOf default color (#904)

# Description
Test fails because emmo has added a relation.

## Type of change
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- [x] Bug fix.
- [ ] New feature.
- [ ] Documentation update.
- [ ] Test update.

## Checklist
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after creating the PR. -->

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- [ ] 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
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Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: francescalb

ontopy/graph.py

@@ -112,7 +112,9 @@
         "hasConvention": {"color": "orange", "style": "dashed"},
         "hasProperty": {"color": "orange", "style": "dotted"},
         "hasOutput": {"color": "darkviolet", "style": "dotted"},
+        "isOutputOf": {"color": "darkviolet", "style": "dotted"},
         "hasInput": {"color": "darkviolet"},
+        "inInputTo": {"color": "darkviolet"},
         "isQuantityValueFor": {"color": "green"},
         "hasTemporaryParticipant": {"color": "darkviolet", "style": "dashed"},
         "refersTo": {"color": "cyan", "style": "dotted"},