Add if_python3 support

Reviewed-by: Michael Reed <m.reed@mykolab.com>, Daniel Hahler <github@thequod.de>
Helped-by: Daniel Hahler <github@thequod.de>

runtime/autoload/provider/python3.vim

@@ -0,0 +1,47 @@
+" The Python3 provider uses a Python3 host to emulate an environment for running
+" python3 plugins. See ":help nvim-provider" for more information.
+"
+" Associating the plugin with the Python3 host is the first step because
+" plugins will be passed as command-line arguments
+
+if exists('g:loaded_python3_provider')
+  finish
+endif
+let g:loaded_python3_provider = 1
+
+let [s:prog, s:err] = provider#pythonx#Detect(3)
+if s:prog == ''
+  " Detection failed
+  finish
+endif
+
+function! provider#python3#Prog()
+  return s:prog
+endfunction
+
+function! provider#python3#Error()
+  return s:err
+endfunction
+
+let s:plugin_path = expand('<sfile>:p:h').'/script_host.py'
+
+" The Python3 provider plugin will run in a separate instance of the Python3
+" host.
+call remote#host#RegisterClone('legacy-python3-provider', 'python3')
+call remote#host#RegisterPlugin('legacy-python3-provider', s:plugin_path, [])
+
+function! provider#python3#Call(method, args)
+  if !exists('s:host')
+    let s:rpcrequest = function('rpcrequest')
+
+    " Ensure that we can load the Python3 host before bootstrapping
+    try
+      let s:host = remote#host#Require('legacy-python3-provider')
+    catch
+      echomsg v:exception
+      finish
+    endtry
+  endif
+
+  return call(s:rpcrequest, insert(insert(a:args, 'python_'.a:method), s:host))
+endfunction