Docs: Clarify that Windows is not supported; fix Sphinx build (horovod#3377)

Signed-off-by: Max H. Gerlach <[email protected]>

Author: maxhgerlach

README.rst

@@ -115,7 +115,7 @@ scale.
 
 Install
 -------
-To install Horovod:
+To install Horovod on Linux or macOS:
 
 1. Install `CMake <https://cmake.org/install/>`__
 

docs/install.rst

@@ -6,6 +6,7 @@ Horovod Installation Guide
 Requirements
 ------------
 
+- GNU Linux or macOS
 - Python >= 3.6
 - `g++-5` or above, or another compiler supporting C++14
 - CMake 3.13 or newer
@@ -19,6 +20,8 @@ For best performance on GPU:
 If Horovod in unable to find the CMake binary, you may need to set ``HOROVOD_CMAKE`` in your environment before
 installing.
 
+Horovod does not support Windows.
+
 Frameworks
 ----------
 

docs/mocks.py

@@ -38,6 +38,7 @@ def _dummy():
     'psutil',
 
     'fsspec',
+    'fsspec.callbacks',
     'fsspec.core',
     'fsspec.utils',
 

docs/summary.rst

@@ -107,7 +107,7 @@ scale.
 
 Install
 -------
-To install Horovod:
+To install Horovod on Linux or macOS:
 
 1. Install `CMake <https://cmake.org/install/>`__