Bump pyyaml from 3.12 to 5.4 in /docs

docs/requirements.txt

@@ -4,28 +4,65 @@
 #
 #    pip-compile --output-file requirements.txt requirements.in
 #
-alabaster==0.7.10         # via sphinx
-argh==0.26.2              # via sphinx-autobuild, watchdog
-babel==2.4.0              # via sphinx
-backports-abc==0.5        # via tornado
-certifi==2017.4.17        # via tornado
-commonmark==0.5.4         # via recommonmark
-docutils==0.13.1          # via recommonmark, sphinx
-imagesize==0.7.1          # via sphinx
-Jinja2==2.9.6             # via sphinx
-livereload==2.5.1         # via sphinx-autobuild
-MarkupSafe==1.0           # via jinja2
-pathtools==0.1.2          # via sphinx-autobuild, watchdog
-port-for==0.3.1           # via sphinx-autobuild
-Pygments==2.2.0           # via sphinx
-pytz==2017.2              # via babel
-PyYAML==3.12              # via sphinx-autobuild, watchdog
+alabaster==0.7.10
+    # via sphinx
+argh==0.26.2
+    # via
+    #   sphinx-autobuild
+    #   watchdog
+babel==2.4.0
+    # via sphinx
+backports-abc==0.5
+    # via tornado
+certifi==2017.4.17
+    # via tornado
+commonmark==0.5.4
+    # via recommonmark
+docutils==0.13.1
+    # via
+    #   recommonmark
+    #   sphinx
+imagesize==0.7.1
+    # via sphinx
+jinja2==2.9.6
+    # via sphinx
+livereload==2.5.1
+    # via sphinx-autobuild
+markupsafe==1.0
+    # via jinja2
+pathtools==0.1.2
+    # via
+    #   sphinx-autobuild
+    #   watchdog
+port-for==0.3.1
+    # via sphinx-autobuild
+pygments==2.2.0
+    # via sphinx
+pytz==2017.2
+    # via babel
+pyyaml==5.4
+    # via
+    #   sphinx-autobuild
+    #   watchdog
 recommonmark==0.4.0
-requests==2.13.0          # via sphinx
-singledispatch==3.4.0.3   # via tornado
-six==1.10.0               # via livereload, singledispatch, sphinx
-snowballstemmer==1.2.1    # via sphinx
+    # via -r requirements.in
+requests==2.13.0
+    # via sphinx
+singledispatch==3.4.0.3
+    # via tornado
+six==1.10.0
+    # via
+    #   livereload
+    #   sphinx
+snowballstemmer==1.2.1
+    # via sphinx
 sphinx-autobuild==0.6.0
+    # via -r requirements.in
 sphinx==1.5.5
-tornado==4.5.1            # via livereload, sphinx-autobuild
-watchdog==0.8.3           # via sphinx-autobuild
+    # via -r requirements.in
+tornado==4.5.1
+    # via
+    #   livereload
+    #   sphinx-autobuild
+watchdog==0.8.3
+    # via sphinx-autobuild