Bump babel from 2.3.4 to 2.9.1

Bumps [babel](https://github.com/python-babel/babel) from 2.3.4 to 2.9.1.
- [Release notes](https://github.com/python-babel/babel/releases)
- [Changelog](https://github.com/python-babel/babel/blob/master/CHANGES)
- [Commits](python-babel/babel@2.3.4...v2.9.1)

---
updated-dependencies:
- dependency-name: babel
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

dev-requirements.txt

@@ -4,38 +4,73 @@
 #
 #    pip-compile --output-file dev-requirements.txt dev-requirements.in
 #
-
-alabaster==0.7.8          # via sphinx
-argh==0.26.2              # via watchdog
-babel==2.3.4              # via sphinx
+alabaster==0.7.8
+    # via sphinx
+argh==0.26.2
+    # via watchdog
+babel==2.9.1
+    # via sphinx
 bumpversion==0.5.3
+    # via -r dev-requirements.in
 check-manifest==0.31
-click==6.6                # via pip-tools
+    # via -r dev-requirements.in
+click==6.6
+    # via pip-tools
 coverage==4.1
-Cython==0.24
-docutils==0.12            # via sphinx
-enum34==1.1.6             # via hypothesis
-first==2.0.1              # via pip-tools
+    # via -r dev-requirements.in
+cython==0.24
+    # via -r dev-requirements.in
+docutils==0.12
+    # via sphinx
+first==2.0.1
+    # via pip-tools
 flake8==2.5.4
+    # via -r dev-requirements.in
 hypothesis==3.4.0
-imagesize==0.7.1          # via sphinx
-Jinja2==2.8               # via sphinx
-MarkupSafe==0.23          # via jinja2
-mccabe==0.4.0             # via flake8
-pathtools==0.1.2          # via watchdog
-pep8==1.7.0               # via flake8
+    # via -r dev-requirements.in
+imagesize==0.7.1
+    # via sphinx
+jinja2==2.8
+    # via sphinx
+markupsafe==0.23
+    # via jinja2
+mccabe==0.4.0
+    # via flake8
+pathtools==0.1.2
+    # via watchdog
+pep8==1.7.0
+    # via flake8
 pip-tools==1.6.5
-pluggy==0.3.1             # via tox
-py==1.4.31                # via pytest, tox
-pyflakes==1.0.0           # via flake8
-Pygments==2.1.3           # via sphinx
+    # via -r dev-requirements.in
+pluggy==0.3.1
+    # via tox
+py==1.4.31
+    # via
+    #   pytest
+    #   tox
+pyflakes==1.0.0
+    # via flake8
+pygments==2.1.3
+    # via sphinx
 pytest==2.9.1
-pytz==2016.4              # via babel
-PyYAML==3.11              # via watchdog
-six==1.10.0               # via pip-tools, sphinx
-snowballstemmer==1.2.1    # via sphinx
-Sphinx==1.4.1
+    # via -r dev-requirements.in
+pytz==2016.4
+    # via babel
+pyyaml==3.11
+    # via watchdog
+six==1.10.0
+    # via
+    #   pip-tools
+    #   sphinx
+snowballstemmer==1.2.1
+    # via sphinx
+sphinx==1.4.1
+    # via -r dev-requirements.in
 tox==2.3.1
-virtualenv==15.0.1        # via tox
+    # via -r dev-requirements.in
+virtualenv==15.0.1
+    # via tox
 watchdog==0.8.3
+    # via -r dev-requirements.in
 wheel==0.29.0
+    # via -r dev-requirements.in