minor #11427 Updated a dependency for security reasons (javiereguiluz)

This PR was merged into the 3.4 branch.

Discussion
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Updated a dependency for security reasons

GitHub just alerted us about this:

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Commits
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edc1259 Updated a dependency for security reasons

Author: javiereguiluz

_build/.requirements.txt

@@ -2,7 +2,7 @@ alabaster==0.7.10
 Babel==2.4.0
 docutils==0.13.1
 imagesize==0.7.1
-Jinja2==2.9.6
+Jinja2==2.10.1
 MarkupSafe==1.0
 Pygments==2.2.0
 pytz==2017.2