Clarify that we do not support python 3.7 (#512)

* Update setup.py

* Readme: no python 3.7

* specify 3.6 in venv

* clarify 3.7 error

we will support it eventually

setup.py

@@ -1,4 +1,13 @@
 from setuptools import setup, find_packages
+import sys
+
+if sys.version_info[0:2] != (3, 6):
+    raise ImportError(
+        "cellxgene currently only supports python 3.6. Python 3.7 is known to fail; we will look at supporting "
+        "versions other than 3.6 in the future."
+        "See https://github.com/chanzuckerberg/cellxgene#conda-and-virtual-environments "
+        "for more help with installation."
+    )
 
 with open("README.md", "rb") as fh:
     long_description = fh.read().decode()
@@ -16,19 +25,24 @@ setup(
     author_email="cweaver@chanzuckerberg.com",
     description="Web application for exploration of large scale scRNA-seq datasets",
     long_description=long_description,
-    long_description_content_type='text/markdown',
+    long_description_content_type="text/markdown",
     install_requires=requirements,
     include_package_data=True,
     zip_safe=False,
-    classifiers=(
-        "Programming Language :: Python :: 3",
+    classifiers=[
+        "Framework :: Flask",
+        "Intended Audience :: Science/Research",
         "License :: OSI Approved :: MIT License",
-    ),
-    entry_points={
-        "console_scripts":
-            ["cellxgene = server.cli.cli:cli"]
-    },
-    extras_require=dict(
-        louvain=['python-igraph', 'louvain>=0.6'],
-    ),
+        "Natural Language :: English",
+        "Operating System :: POSIX",
+        "Operating System :: Unix",
+        "Operating System :: MacOS :: MacOS X",
+        "Programming Language :: JavaScript",
+        "Programming Language :: Python :: 3",
+        "Programming Language :: Python :: 3.6",
+        "Programming Language :: Python :: 3 :: Only",
+        "Topic :: Scientific/Engineering :: Bio-Informatics",
+    ],
+    entry_points={"console_scripts": ["cellxgene = server.cli.cli:cli"]},
+    extras_require=dict(louvain=["python-igraph", "louvain>=0.6"]),
 )