Add Scrapy dependencies back to docs/requirements.txt

docs/requirements.txt

@@ -2,3 +2,18 @@ Sphinx>=2.1
 sphinx-hoverxref
 sphinx-notfound-page
 sphinx_rtd_theme
+
+# Required for ReadTheDocs
+# Keep in sync with setup.py
+Twisted>=17.9.0
+cryptography>=2.0
+cssselect>=0.9.1
+lxml>=3.5.0
+parsel>=1.5.0
+PyDispatcher>=2.0.5
+pyOpenSSL>=16.2.0
+queuelib>=1.4.2
+service_identity>=16.0.0
+w3lib>=1.17.0
+zope.interface>=4.1.3
+protego>=0.1.15

setup.py

@@ -62,6 +62,7 @@ setup(
         'Topic :: Software Development :: Libraries :: Python Modules',
     ],
     python_requires='>=3.5',
+    # Keep in sync with docs/requirements.txt
     install_requires=[
         'Twisted>=17.9.0',
         'cryptography>=2.0',