Fixing doc build by pinning alabaster

[codecov-commenter]

Codecov Report

Base: 76.33% // Head: 76.33% // No change to project coverage :thumbsup:

Coverage data is based on head (bfed65e) compared to base (11d9966). Patch has no changes to coverable lines.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2805 +/- ## ======================================= Coverage 76.33% 76.33% ======================================= Files 88 88 Lines 9672 9672 Branches 2062 2062 ======================================= Hits 7383 7383 Misses 2021 2021 Partials 268 268 ``` | [Impacted Files](https://codecov.io/gh/graphite-project/graphite-web/pull/2805?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=graphite-project) | Coverage Δ | | |---|---|---| | [webapp/graphite/tags/redis.py](https://codecov.io/gh/graphite-project/graphite-web/pull/2805?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=graphite-project#diff-d2ViYXBwL2dyYXBoaXRlL3RhZ3MvcmVkaXMucHk=) | `87.03% <0.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=graphite-project). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=graphite-project)

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[deniszh]

💚 All backports created successfully

Status	Branch	Result
✅	1.1.x

Questions ?

Please refer to the Backport tool documentation