Update dependency python-dotenv to v0.21.1

Author: renovate[bot]

requirements.txt

@@ -10,7 +10,7 @@ PyMySQL==0.10.1
 pyparsing==2.4.7
 pytest==5.4.3
 python-dateutil==2.8.1
-python-dotenv==0.13.0
+python-dotenv==0.21.1
 pytz==2020.1
 six==1.15.0
 SQLAlchemy==1.3.20