Put in a special case for Ubuntu 18.04 and Debian 10 with Python 3.

Fixes #114.

lib/poise_python/python_providers/system.rb

@@ -57,9 +57,22 @@ def python_binary
 
       def install_python
         install_system_packages
+        if node.platform_family?('debian') && system_package_name == 'python3.6'
+          # Ubuntu 18.04 and Debian 10 have some weird dependency fuckery going on.
+          _options = options
+          package %w{python3.6-venv python3.6-distutils} do
+            action(:upgrade) if _options['package_upgrade']
+          end
+        end
       end
 
       def uninstall_python
+        if node.platform_family?('debian') && system_package_name == 'python3.6'
+          # Other side of the depdency nonsense.
+          package %w{python3.6-venv python3.6-distutils} do
+            action(:purge)
+          end
+        end
         uninstall_system_packages
       end
 
@@ -74,7 +87,7 @@ def system_package_candidates(version)
           end
           # Aliases for 2 and 3.
           if version == '3' || version == ''
-            names.concat(%w{python3.5 python35 python3.4 python34 python3.3 python33 python3.2 python32 python3.1 python31 python3.0 python30 python3})
+            names.concat(%w{python3.7 python37 python3.6 python36 python3.5 python35 python3.4 python34 python3.3 python33 python3.2 python32 python3.1 python31 python3.0 python30 python3})
           end
           if version == '2' || version == ''
             names.concat(%w{python2.7 python27 python2.6 python26 python2.5 python25})

test/spec/python_providers/system_spec.rb

@@ -43,7 +43,7 @@
 
   context 'with version ""' do
     let(:python_version) { '' }
-    it_behaves_like 'system provider', %w{python3.5 python35 python3.4 python34 python3.3 python33 python3.2 python32 python3.1 python31 python3.0 python30 python3 python2.7 python27 python2.6 python26 python2.5 python25 python}, 'python3.4'
+    it_behaves_like 'system provider', %w{python3.7 python37 python3.6 python36 python3.5 python35 python3.4 python34 python3.3 python33 python3.2 python32 python3.1 python31 python3.0 python30 python3 python2.7 python27 python2.6 python26 python2.5 python25 python}, 'python3.4'
   end # /context with version ""
 
   context 'with version 2' do
@@ -53,7 +53,7 @@
 
   context 'with version 3' do
     let(:python_version) { '3' }
-    it_behaves_like 'system provider', %w{python3.5 python35 python3.4 python34 python3.3 python33 python3.2 python32 python3.1 python31 python3.0 python30 python3 python}, 'python3.4'
+    it_behaves_like 'system provider', %w{python3.7 python37 python3.6 python36 python3.5 python35 python3.4 python34 python3.3 python33 python3.2 python32 python3.1 python31 python3.0 python30 python3 python}, 'python3.4'
   end # /context with version 3
 
   context 'with version 2.3' do
@@ -65,6 +65,13 @@
     it_behaves_like 'system provider', %w{python2.3 python23 python}, 'python2.3'
   end # /context with version 2.3
 
+  context 'on Ubuntu 18.04' do
+    let(:chefspec_options) { {platform: 'ubuntu', version: '18.04'} }
+    let(:python_version) { '3.6' }
+
+    it { is_expected.to install_package(%w{python3.6-venv python3.6-distutils}) }
+  end # /context on Ubuntu 18.04
+
   context 'on Debian 8' do
     before { chefspec_options.update(platform: 'debian', version: '8.9') }
     it_behaves_like 'system provider', 'python3.4'