Make gevent mandatory for install

We use it in the CLI tools and it's pretty expected here.

setup.py

@@ -13,12 +13,17 @@
     packages=find_packages(exclude=["tests", "tests.*"]),
     python_requires=">=3.6",
     setup_requires=["setuptools_scm"],
-    install_requires=["posix_ipc>=1.0.0", "pyjwt>=1.6.0", "requests>=2.21.0", "thrift>=0.12.0"],
+    install_requires=[
+        "posix_ipc>=1.0.0",
+        "pyjwt>=1.6.0",
+        "requests>=2.21.0",
+        "thrift>=0.12.0",
+        "gevent>=1.3",
+    ],
     extras_require={
         "amqp": ["kombu>=4.0.0"],
         "cassandra": ["cassandra-driver>=3.13.0"],
         "cqlmapper": ["cqlmapper>=0.1.0"],
-        "gevent": ["gevent>=1.3"],
         "memcache": ["pymemcache>=1.3.0,<=2.0.0"],
         "pyramid": ["pyramid>=1.9.0"],
         "redis": ["redis>=2.10.0,<=3.0.0"],