Update dependency python-dotenv to v0.15.0

Author: renovate-bot

requirements.txt

@@ -32,7 +32,7 @@ py==1.9.0
 pyparsing==2.4.7
 pytest==6.1.1
 python-dateutil==2.8.1
-python-dotenv==0.14.0
+python-dotenv==0.15.0
 pytz==2020.1
 regex==2020.10.15
 retrying==1.3.3