Fix sphinx documentation tools

[pieths]

The tools which are used to build the documentation in src\python\docs\sphinx are no longer working.

Here are a few of the errors which are produced when building the documentation:

ERROR: sphinx-docfx-yaml 1.2.71 has requirement sphinx==1.5.5, but you'll have sphinx 2.1.1 which is incompatible.

Traceback (most recent call last):
  File "C:\Users\v-pischo\dev\nimbusml\docs-csr\src\python\docs\sphinx\conf.py", line 30, in install_and_import
    importlib.import_module(package)
  File "C:\Users\v-pischo\dev\nimbusml\docs-csr\dependencies\Python3.6\lib\importlib\__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 994, in _gcd_import
  File "<frozen importlib._bootstrap>", line 971, in _find_and_load
  File "<frozen importlib._bootstrap>", line 953, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'sphinxmdoutput'

WARNING: Field list ends without a blank line; unexpected unindent.

WARNING: Unknown directive type "rxtitle", "rxdescription"

Traceback (most recent call last):
  File "C:\Users\v-pischo\dev\nimbusml\docs-csr\src\python\docs\sphinx\conf.py", line 33, in install_and_import
    pip.main(
AttributeError: module 'pip' has no attribute 'main'

During handling of the above exception, another exception occurred:

And many other warnings that may or may not have an effect on the generated documentation.