Update dependency python-dotenv to v0.15.0

Author: renovate-bot

requirements.txt

@@ -6,7 +6,7 @@ numpy==1.19.1
 pandas==1.1.1
 PyMySQL==0.10.0
 python-dateutil==2.8.1
-python-dotenv==0.14.0
+python-dotenv==0.15.0
 pytz==2020.1
 requests==2.24.0
 six==1.15.0