Fix AV, see issue #741

[Lambdac0re]

• closes ValueError when I try to get data from Alpha Vantage #741
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• passes black --check pandas_datareader
• added entry to docs/source/whatsnew/vLATEST.txt

Can't do the tests etc right now, just doing a quick inline bugfix, sorry about that.

[codecov-io]

Codecov Report

Merging #856 (617ea59) into master (90f155a) will not change coverage.
The diff coverage is 0.00%.

@@ Coverage Diff @@ ## master #856 +/- ## ======================================= Coverage 64.03% 64.03% ======================================= Files 65 65 Lines 2906 2906 Branches 311 311 ======================================= Hits 1861 1861 Misses 969 969 Partials 76 76 

Impacted Files	Coverage Δ
pandas_datareader/av/time_series.py	40.00% <0.00%> (ø)

---

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 64.03%. Comparing base (90f155a) to head (617ea59).
⚠️ Report is 110 commits behind head on main.

Files with missing lines	Patch %	Lines
pandas_datareader/av/time_series.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@ ## main #856 +/- ## ========================================== - Coverage 64.14% 64.03% -0.11%  ========================================== Files 65 65 Lines 2906 2906 Branches 311 311 ========================================== - Hits 1864 1861 -3  - Misses 967 969 +2  - Partials 75 76 +1 

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