limit urllib3 sub-dependency

* dropped python <=3.6
* incompatibility with vcrpy

requirements.txt

@@ -1,6 +1,7 @@
 six>=1.10.0
 pycryptodomex>=3.7.0
 requests>=2.9.1
+urllib3<2
 vdf>=3.3
 gevent>=1.3.0
 protobuf~=3.0; python_version >= '3'

setup.py

@@ -17,6 +17,7 @@ install_requires = [
     'six>=1.10',
     'pycryptodomex>=3.7.0',
     'requests>=2.9.1',
+    'urllib3<2',
     'vdf>=3.3',
     'cachetools>=3.0.0',
     "win-inet-pton; python_version == '2.7' and sys_platform == 'win32'",