Upgrade alpha_vantage to 1.6.0

[fabaff]

Description:

• Pandas is no longer a hard requirement
• Some bug fixes on pandas indexing
• Better unit testing

Related issue (if applicable): fixes #11223

Example entry for configuration.yaml (if applicable):

sensor:
  - platform: alpha_vantage
    api_key: !secret av_api

Checklist:

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

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• New dependencies have been added to the REQUIREMENTS variable ([example][ex-requir]).
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.