fix: requirements_dev.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-PYGMENTS-1086606
- https://snyk.io/vuln/SNYK-PYTHON-PYGMENTS-1088505

Author: snyk-bot

requirements_dev.txt

@@ -4,4 +4,5 @@ twine>=1.13.0
 black>=18.9b0
 pytest>=4.3.0
 pytest-cov>=2.6.1
-coveralls>=1.6.0
+coveralls>=1.6.0
+pygments>=2.7.4 # not directly required, pinned by Snyk to avoid a vulnerability