fix: rationalize platform constraints for 'pyarrow' extra (#235)

Release-As: 1.27.2

Author: tseaver

setup.py

@@ -47,13 +47,10 @@
     ],
     "pandas": ["pandas>=0.17.1"],
     # Exclude PyArrow dependency from Windows Python 2.7.
-    'pyarrow: platform_system == "Windows"': [
-        "pyarrow>=1.0.0, <2.0dev; python_version>='3.5'",
-    ],
-    'pyarrow: platform_system != "Windows"': [
-        "pyarrow>=1.0.0, <2.0dev; python_version>='3.5'",
+    "pyarrow": [
+        "pyarrow >= 1.0.0, < 2.0dev; python_version >= '3.5'",
         # Pyarrow >= 0.17.0 is not compatible with Python 2 anymore.
-        "pyarrow < 0.17.0; python_version < '3.0'",
+        "pyarrow < 0.17.0; python_version < '3.0' and platform_system != 'Windows'",
     ],
     "tqdm": ["tqdm >= 4.0.0, <5.0.0dev"],
     "fastparquet": [