text: Title: Cannot install horovod[spark] for Tensorflow 2.6
  Body: **Environment:**
  1. Framework: TensorFlow
  2. Framework version:2.6.2
  3. Horovod version: 0.23
  4. MPI version:4.1.1
  5. CUDA version:N/A
  6. NCCL version:N/A
  7. Python version: 3.7
  8. Spark / PySpark version: 2.4.5
  9. Ray version:N/A
  10. OS and version: RHEL 8.4
  11. GCC version: 9.3.0
  12. CMake version: 3.5.0
  
  **Checklist:**
  1. Did you search issues to find if somebody asked this question before?  Yes
  2. If your question is about hang, did you read [this doc](https://github.com/horovod/horovod/blob/master/docs/running.rst)? N/A
  3. If your question is about docker, did you read [this doc](https://github.com/horovod/horovod/blob/master/docs/docker.rst)? N/A
  4. Did you check if you question is answered in the [troubleshooting guide] (https://github.com/horovod/horovod/blob/master/docs/troubleshooting.rst)? Yes
  
  **Bug report:**
  Please describe erroneous behavior you're observing and steps to reproduce it.
  
  ```
  Installing collected packages: pyparsing, pycparser, pyzmq, pyyaml, pyarrow, psutil, packaging, future, fsspec, diskcache, dill, cloudpickle, cffi, petastorm, horovod, h5py
    Attempting uninstall: h5py
      Found existing installation: h5py 3.1.0
      Uninstalling h5py-3.1.0:
        Successfully uninstalled h5py-3.1.0
  ERROR: pip's dependency resolver does not currently take into account all the packages that are installed. This behaviour is the source of the following dependency conflicts.
  tensorflow 2.6.2 requires h5py~=3.1.0, but you have h5py 2.10.0 which is incompatible.
  ```
  
  **Reproduce Steps:**
  
  1. `conda create -n horovod python=3.7`
  2. `conda activate horovod`
  3. `conda install pyspark=2.4.5 openmpi-mpicc cmake -c conda-forge`
  4. `pip install tensorflow==2.6.2`
  5. `HOROVOD_WITH_MPI=1 HOROVOD_WITH_TENSORFLOW=1 pip install horovod[spark]`
  