Bump urllib3 from 1.25.3 to 1.25.8 in /sourcedocs

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.25.3 to 1.25.8.
- [Release notes](https://github.com/urllib3/urllib3/releases)
- [Changelog](https://github.com/urllib3/urllib3/blob/main/CHANGES.rst)
- [Commits](urllib3/urllib3@1.25.3...1.25.8)

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

sourcedocs/requirements.txt

@@ -4,31 +4,56 @@
 #
 #    pip-compile sourcedocs/requirements.in
 #
-alabaster==0.7.12         # via sphinx
-attrs==19.1.0             # via packaging
-babel==2.7.0              # via sphinx
-certifi==2019.6.16        # via requests
-chardet==3.0.4            # via requests
-docutils==0.15.2          # via sphinx
-idna==2.8                 # via requests
-imagesize==1.1.0          # via sphinx
-jinja2==2.10.1            # via sphinx
-markupsafe==1.1.1         # via jinja2
-packaging==19.1           # via sphinx
-pygments==2.4.2           # via sphinx
-pyparsing==2.4.2          # via packaging
-pytz==2019.2              # via babel
-requests==2.22.0          # via sphinx
-six==1.12.0               # via packaging
-snowballstemmer==1.9.1    # via sphinx
-sphinx==3.3.1             # via -r sourcedocs/requirements.in
-sphinxcontrib-applehelp==1.0.1  # via sphinx
-sphinxcontrib-devhelp==1.0.1  # via sphinx
-sphinxcontrib-htmlhelp==1.0.2  # via sphinx
-sphinxcontrib-jsmath==1.0.1  # via sphinx
-sphinxcontrib-qthelp==1.0.2  # via sphinx
-sphinxcontrib-serializinghtml==1.1.3  # via sphinx
-urllib3==1.25.3           # via requests
+alabaster==0.7.12
+    # via sphinx
+attrs==19.1.0
+    # via packaging
+babel==2.7.0
+    # via sphinx
+certifi==2019.6.16
+    # via requests
+chardet==3.0.4
+    # via requests
+docutils==0.15.2
+    # via sphinx
+idna==2.8
+    # via requests
+imagesize==1.1.0
+    # via sphinx
+jinja2==2.10.1
+    # via sphinx
+markupsafe==1.1.1
+    # via jinja2
+packaging==19.1
+    # via sphinx
+pygments==2.4.2
+    # via sphinx
+pyparsing==2.4.2
+    # via packaging
+pytz==2019.2
+    # via babel
+requests==2.22.0
+    # via sphinx
+six==1.12.0
+    # via packaging
+snowballstemmer==1.9.1
+    # via sphinx
+sphinx==3.3.1
+    # via -r requirements.in
+sphinxcontrib-applehelp==1.0.1
+    # via sphinx
+sphinxcontrib-devhelp==1.0.1
+    # via sphinx
+sphinxcontrib-htmlhelp==1.0.2
+    # via sphinx
+sphinxcontrib-jsmath==1.0.1
+    # via sphinx
+sphinxcontrib-qthelp==1.0.2
+    # via sphinx
+sphinxcontrib-serializinghtml==1.1.3
+    # via sphinx
+urllib3==1.25.8
+    # via requests
 
 # The following packages are considered to be unsafe in a requirements file:
 # setuptools