Update dependency python-dotenv to v0.19.1 (#76)

Author: renovate[bot]

requirements.txt

@@ -20,7 +20,7 @@ pandas==1.2.0; python_full_version >= "3.7.1"
 plotly==4.14.1; python_version >= "2.7" and python_full_version < "3.0.0" or python_full_version >= "3.3.0"
 pymysql==1.0.1; python_version >= "3.6"
 python-dateutil==2.8.1; python_full_version >= "3.7.1"
-python-dotenv==0.15.0
+python-dotenv==0.19.1
 pytz==2020.5; python_full_version >= "3.7.1"
 redis==3.5.3; (python_version >= "2.7" and python_full_version < "3.0.0") or (python_full_version >= "3.5.0")
 retrying==1.3.3; python_version >= "2.7" and python_full_version < "3.0.0" or python_full_version >= "3.3.0"