minor symfony#10939 Updated a dependency of the build system (javiereguiluz)

This PR was merged into the 3.4 branch.

Discussion
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Updated a dependency of the build system

This was alerted to the SymfonyCorp team via GitHub:

![dep-security](https://user-images.githubusercontent.com/73419/51902931-6ca32b00-23bb-11e9-826c-b36df3cc1759.png)

Commits
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49d78fc Updated a dependency of the build system

Author: javiereguiluz

_build/.requirements.txt

@@ -6,7 +6,7 @@ Jinja2==2.9.6
 MarkupSafe==1.0
 Pygments==2.2.0
 pytz==2017.2
-requests==2.12.5
+requests==2.20.0
 six==1.10.0
 snowballstemmer==1.2.1
 Sphinx==1.3.6