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/pythonx.vim

@@ -0,0 +1,69 @@
+" The Python provider helper
+if exists('s:loaded_pythonx_provider')
+  finish
+endif
+
+let s:loaded_pythonx_provider = 1
+
+function! provider#pythonx#Detect(ver) abort
+  let host_var = (a:ver == 2) ?
+        \ 'g:python_host_prog' : 'g:python3_host_prog'
+  let skip_var = (a:ver == 2) ?
+        \ 'g:python_host_skip_check' : 'g:python3_host_skip_check'
+  let skip = exists(skip_var) ? {skip_var} : 0
+  if exists(host_var)
+    " Disable auto detection
+    let [check, err] = s:check_interpreter({host_var}, a:ver, skip)
+    return check ? [{host_var}, err] : ['', err]
+  endif
+
+  let detect_versions = (a:ver == 2) ?
+        \   ['2.7', '2.6', '2', '']
+        \ : ['3.5', '3.4', '3.3', '3.2', '3', '']
+
+  for prog in map(detect_versions, "'python' . v:val")
+    let [check, err] = s:check_interpreter(prog, a:ver, skip)
+    if check
+      let [check, err] = s:check_version(prog, a:ver, skip)
+      return [prog, err]
+    endif
+  endfor
+
+  " No Python interpreter
+  return ['', 'Neovim module installed Python'
+        \ .a:ver.' interpreter is not found.']
+endfunction
+
+function! s:check_version(prog, ver, skip) abort
+  if a:skip
+    return [1, '']
+  endif
+
+  let get_version =
+        \ ' -c "import sys; sys.stdout.write(str(sys.version_info[0]) + '.
+        \ '\".\" + str(sys.version_info[1]))"'
+  let min_version = (a:ver == 2) ? '2.6' : '3.3'
+  if system(a:prog . get_version) >= min_version
+    return [1, '']
+  endif
+  return [0, 'Python ' . get_version . ' interpreter is not supported.']
+endfunction
+
+function! s:check_interpreter(prog, ver, skip) abort
+  if !executable(a:prog)
+    return [0, 'Python'.a:ver.' interpreter is not executable.']
+  endif
+
+  if a:skip
+    return [1, '']
+  endif
+
+  " Load neovim module check
+  call system(a:prog . ' -c ' .
+        \ (a:ver == 2 ?
+        \   '''import pkgutil; exit(pkgutil.get_loader("neovim") is None)''':
+        \   '''import importlib; exit(importlib.find_loader("neovim") is None)''')
+        \ )
+  return [!v:shell_error, 'Python'.a:ver.' interpreter have not neovim module.']
+endfunction
+