Bump Babel to fix security issue

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  |  by pyup.io                                              \______/            |
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  +==============================================================================+
  | REPORT                                                                       |
  | checked 25 packages, using free DB (updated once a month)                    |
  +============================+===========+==========================+==========+
  | package                    | installed | affected                 | ID       |
  +============================+===========+==========================+==========+
  | babel                      | 2.8.0     | <2.9.1                   | 42203    |
  +==============================================================================+
  | Babel 2.9.1 includes a fix for CVE-2021-42771: Babel.Locale in Babel before  |
  | 2.9.1 allows attackers to load arbitrary locale .dat files (containing       |
  | serialized Python objects) via directory traversal, leading to code          |
  | execution.                                                                   |
  | python-babel/babel#782                               |
  | https://lists.debian.org/debian-lts/2021/10/msg00040.html                    |
  | https://www.tenable.com/security/research/tra-2021-14                        |
  | https://lists.debian.org/debian-lts-announce/2021/10/msg00018.html           |
  +==============================================================================+

doc/Pipfile

@@ -9,7 +9,7 @@ Pygments = "==2.4.0"
 Sphinx = "==3.0.4"
 sphinx-prompt = "==1.1.0"
 # Lock
-babel = "==2.8.0"
+babel = "==2.9.1"
 certifi = "==2019.11.28"
 chardet = "==3.0.4"
 docutils = "==0.16"