Docs: Clean up docs/requirements.txt. It just needs Sphinx and friends.

docs/requirements.txt

@@ -1,48 +1,7 @@
 alabaster==0.7.12
-appdirs==1.4.3
-atomicwrites==1.2.1
-attrs==18.2.0
-babel==2.6.0
-black==18.9b0
-bleach==3.1.4
-certifi==2018.8.24
-cffi==1.11.5
-chardet==3.0.4
-click==7.0
-cmarkgfm==0.4.2
-colorama==0.4.0 ; sys_platform == 'win32'
 docutils==0.14
-flake8==3.5.0
-flask==1.0.2
-future==0.16.0
-idna==2.7
-imagesize==1.1.0
-itsdangerous==0.24
 jinja2==2.10
 markupsafe==1.0
-mccabe==0.6.1
-more-itertools==4.3.0
-packaging==18.0
-pkginfo==1.4.2
-pluggy==0.7.1
-py==1.7.0
-pycodestyle==2.3.1
-pycparser==2.19
-pyflakes==1.6.0
-pygments==2.2.0
-pyparsing==2.2.2
-pytest==3.8.2
-pytz==2018.5
 readme-renderer==22.0
-requests-toolbelt==0.8.0
-requests==2.19.1
-six==1.11.0
-snowballstemmer==1.2.1
 sphinx==1.8.1
 sphinxcontrib-websupport==1.1.0
-toml==0.10.0
-tqdm==4.26.0
-twine==1.12.1
-urllib3==1.23
-webencodings==0.5.1
-werkzeug==0.15.5