Revert "Assume standard Python3/pip3 install of docutils (#610)"

This reverts commit 31bfe43.

Turns out that on some RHEL8 machines, 'rst2thml' is installed, but not
'rst2html.py'.  For example, for the hpws machines at SNL, /usr/bin/rst2html
is installed with version:

  $ rst2html --version
  rst2html (Docutils 0.14, Python 3.6.8, on linux)

So this logic looking for 'rst2thml' and 'rst2html.py' is still useful.

tribits/python_utils/GenerateDocUtilsOutput.py

@@ -39,9 +39,13 @@ def generateFile(filePath, generateCmnd, outFile=None, workingDir="",
 
 def addCmndLineOptions(clp):
 
-  # Assume standard install of docutils with Python3
-  rst2html = "rst2html.py"
-  rst2latex = "rst2latex.py"
+  # Find the right default for the current system
+  rst2html = "rst2html"
+  rst2latex = "rst2latex"
+  rst2htmlWhich = getCmndOutput("which rst2html", True, False)
+  if rst2htmlWhich == "" or re.match(".+no rst2html.+", rst2htmlWhich):
+    rst2html = rst2html+".py"
+    rst2latex = rst2latex+".py"
 
   clp.add_option(
     "--file-base", dest="fileBase", type="string",