Drop keepalive from requirements.txt (Closes: #846871)

As keepalive support is a weak dependency (absence makes keepalive
enabling only raise a warning and otherwise a no-op), listing it as a
dependency causes trouble with test setups. Demoting it to an extra
(feature) dependency.

Author: chrysn

requirements.txt

@@ -1,2 +1 @@
 rdflib>=4.0
-keepalive>=0.5

setup.py

@@ -57,6 +57,9 @@
       platforms = ['any'],
       packages = ['SPARQLWrapper'],
       install_requires = _install_requires,
+      extras_require = {
+        'keepalive': ['keepalive>=0.5'],
+      },
       classifiers =  [
         'Development Status :: 5 - Production/Stable',
         'Intended Audience :: Developers',