doc: split HTML and manpage generation

* Add a 'man' tox environment to build only the manpage.
* Add a dedicated 'man-min' environment to build the manpage with the
  minimum version of Sphinx, that is the one of Debian Buster and that
  will be used to generate the manpage when building the Debian package.
* Let the sphinx tox environment just build the HTML documentation.

Change-Id: Ic1bb2b53720fca4b90fcc061fb00fe8a8b9d9dac

tox.ini

@@ -1,19 +1,22 @@
 [tox]
 minversion = 3.0.0
-envlist = py{37}-{flake8,unit,unit-min,unitbase,bandit,prospector,sphinx}
+envlist = py{37}-{flake8,unit,unit-min,unitbase,bandit,prospector,sphinx,man,man-min}
 skip_missing_interpreters = True
 
 [testenv]
 usedevelop = True
-whitelist_externals = sed
+whitelist_externals =
+    rm
+    sed
 description =
     flake8: Style consistency checker
     unit: Run unit tests
     unitbase: Run unit tests with base dependencies only
     bandit: Security-oriented static analyzer
     prospector: Static analysis multi-tool
-    sphinx: Build documentation and manpages
+    sphinx: Build html documentation
     integration: Run integration tests
+    man: Build the man page
     min: [minimum supported version of dependencies]
     py37: (Python 3.7)
 envdir =
@@ -29,9 +32,11 @@ commands =
     bandit: bandit -l -i -r --skip B101 cumin/tests
     prospector: prospector --profile "{toxinidir}/prospector.yaml" cumin/
     sphinx: python setup.py build_sphinx -b html
-    sphinx: python setup.py build_sphinx -b man
+    man: python setup.py build_sphinx -b man
     # Fix missing space after bold blocks in man page: https://github.com/ribozz/sphinx-argparse/issues/80
-    sphinx: sed -i='' -e 's/^\.B/.B /' '{toxinidir}/doc/build/man/cumin.1'
+    # Use a syntax that works both on BSD/MacOS and Linux
+    man: sed -i.orig -e 's/^\.B/.B /' '{toxinidir}/doc/build/man/cumin.1'
+    man: rm -fv '{toxinidir}/doc/build/man/cumin.1.orig'
     integration: "{toxinidir}/cumin/tests/integration/docker.sh" "transports/clustershell" {posargs}
 deps =
     # Use install_requires and the additional extras_require[NAME] from setup.py