fix: requirements_base.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
- https://snyk.io/vuln/SNYK-PYTHON-PYYAML-559098
- https://snyk.io/vuln/SNYK-PYTHON-PYYAML-590151

requirements_base.txt

@@ -59,7 +59,7 @@ seaborn==0.9.0
 sportsreference==0.4.6
 pydot==1.4.1
 plotly==4.1.0
-PyYAML==5.2
+PyYAML==5.3.1
 twine==2.0.0
 opencv-python==4.1.1.26
 yfinance==0.1.45
@@ -76,3 +76,4 @@ docformatter==1.3.1
 pycodestyle==2.6.0
 yapf==0.30.0
 pyformat==0.7.0
+pygments>=2.7.4 # not directly required, pinned by Snyk to avoid a vulnerability