Bump pygments from 2.3.1 to 2.7.4 in /docs

docs/requirements.txt

@@ -4,23 +4,47 @@
 #
 #    pip-compile docs/requirements.in
 #
-alabaster==0.7.12         # via sphinx
-babel==2.6.0              # via sphinx
-certifi==2019.3.9         # via requests
-chardet==3.0.4            # via requests
-docutils==0.14            # via sphinx
-idna==2.8                 # via requests
-imagesize==1.1.0          # via sphinx
-jinja2==2.10              # via sphinx
-markupsafe==1.1.1         # via jinja2
-packaging==19.0           # via sphinx
-pygments==2.3.1           # via sphinx
-pyparsing==2.3.1          # via packaging
-pytz==2018.9              # via babel
-requests==2.21.0          # via sphinx
-six==1.12.0               # via packaging, sphinx
-snowballstemmer==1.2.1    # via sphinx
+alabaster==0.7.12
+    # via sphinx
+babel==2.6.0
+    # via sphinx
+certifi==2019.3.9
+    # via requests
+chardet==3.0.4
+    # via requests
+docutils==0.14
+    # via sphinx
+idna==2.8
+    # via requests
+imagesize==1.1.0
+    # via sphinx
+jinja2==2.10
+    # via sphinx
+markupsafe==1.1.1
+    # via jinja2
+packaging==19.0
+    # via sphinx
+pygments==2.7.4
+    # via sphinx
+pyparsing==2.3.1
+    # via packaging
+pytz==2018.9
+    # via babel
+requests==2.21.0
+    # via sphinx
+six==1.12.0
+    # via
+    #   packaging
+    #   sphinx
+snowballstemmer==1.2.1
+    # via sphinx
 sphinx-rtd-theme==0.4.3
+    # via -r requirements.in
 sphinx==1.8.5
-sphinxcontrib-websupport==1.1.0  # via sphinx
-urllib3==1.24.1           # via requests
+    # via
+    #   -r requirements.in
+    #   sphinx-rtd-theme
+sphinxcontrib-websupport==1.1.0
+    # via sphinx
+urllib3==1.24.1
+    # via requests