build: Require minimum SciPy version of v1.4.0 (#796)

* Require SciPy v1.4.0 or newer
   - Avoids some 'Iteration limit exceeded' errors and false exits with nans

Author: matthewfeickert

setup.cfg

@@ -38,7 +38,7 @@ packages = find:
 include_package_data = True
 python_requires = >=3.6
 install_requires =
-    scipy  # requires numpy, which is required by pyhf and tensorflow
+    scipy>=1.4.0  # requires numpy, which is required by pyhf and tensorflow
     click>=6.0  # for console scripts,
     tqdm  # for readxml
     jsonschema>=3.2.0  # for utils