Accept to use python3 if python is not on the path

RHEL8 in particular has no python command by design
attempt to use python3 if there is no python command.

https://developers.redhat.com/blog/2018/11/14/python-in-rhel-8/

README.md

@@ -1453,8 +1453,8 @@ class { 'collectd::plugin::protocols':
 
 #### Class: `collectd::plugin::python`
 The plugin uses a fact `python_dir` to find the python load path for modules.
-python must be installed as a pre-requisite for the this fact to give a
-non-default value.
+python or python3 must be installed as a pre-requisite for the this
+fact to give a non-default value.
 
 * `modulepaths` is an array of paths where will be Collectd looking for Python
   modules, Puppet will ensure that each of specified directories exists and it

lib/facter/python_dir.rb

@@ -10,6 +10,12 @@
       else
         Facter::Util::Resolution.exec('python -c "import site; print(site.getsitepackages()[0])"')
       end
+    elsif Facter::Util::Resolution.which('python3')
+      if Facter.value(:osfamily) == 'RedHat'
+        Facter::Util::Resolution.exec('python3 -c "from distutils.sysconfig import get_python_lib; print(get_python_lib())"')
+      else
+        Facter::Util::Resolution.exec('python3 -c "import site; print(site.getsitepackages()[0])"')
+      end
     else
       ''
     end

spec/unit/python_dir_spec.rb

@@ -26,10 +26,23 @@
         expect(Facter.value(:python_dir)).to eq('/usr/lib/python2.7/site-packages')
       end
     end
+
+    context 'RedHat versioned python' do
+      before do
+        Facter.fact(:osfamily).stubs(:value).returns('RedHat')
+        Facter::Util::Resolution.stubs(:which).with('python').returns(false)
+        Facter::Util::Resolution.stubs(:which).with('python3').returns(true)
+        Facter::Util::Resolution.stubs(:exec).with('python3 -c "from distutils.sysconfig import get_python_lib; print(get_python_lib())"').returns('/usr/lib/python3.6/site-packages')
+      end
+      it do
+        expect(Facter.value(:python_dir)).to eq('/usr/lib/python3.6/site-packages')
+      end
+    end
   end
 
   it 'is empty string if python not installed' do
     Facter::Util::Resolution.stubs(:which).with('python').returns(nil)
+    Facter::Util::Resolution.stubs(:which).with('python3').returns(nil)
     expect(Facter.fact(:python_dir).value).to eq('')
   end
 end