[Snyk] Security upgrade networkx from 2.2 to 2.6 (#6)

* fix: requirements.txt to reduce vulnerabilities


The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-NETWORKX-1062709

Note: networkx >= 2.6 has dropped support for Python 3.6

Co-authored-by: Michael R. Crusoe <1330696+mr-c@users.noreply.github.com>

Author: snyk-bot

requirements.txt

@@ -3,3 +3,4 @@ bdbag >= 1.4.1
 arcp >= 0.2.0
 rdflib-jsonld >= 0.4.0, < 0.6.2
 rdflib >= 4.2.2, <6.0
+networkx>=2.6;python_version>"3.6"  # not directly required, pinned by Snyk to avoid a vulnerability