Upgrade alpha_vantage to 2.1.0

[fabaff]

Description:

Commit log: https://github.com/RomelTorres/alpha_vantage/commits/develop

Example entry for configuration.yaml (if applicable):

  - platform: alpha_vantage
    api_key: !secret av_api
    symbols:
      - name: Google
        currency: USD
        symbol: GOOGL

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

If the code communicates with devices, web services, or third-party tools:

• [x ] New dependencies have been added to the REQUIREMENTS variable (example).
• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.