fix(tests): "provider#python3#Prog" was removed upstream #563

Problem:
Tests fail since upstream Nvim commit neovim/neovim@eb5d15e
which removed the (internal) `provider#python3#Prog` function.

    pynvim.api.common.NvimError: nvim_exec2(): Vim(echomsg):E117: Unknown function: provider#python3#Prog

Solution:
Use the new Lua function instead.

test/test_vim.py

@@ -209,10 +209,11 @@ def test_hash(vim: Nvim) -> None:
 
 def test_python3(vim: Nvim) -> None:
     """Tests whether python3 host can load."""
-    python3_prog = vim.command_output('echom provider#python3#Prog()')
-    python3_err = vim.command_output('echom provider#python3#Error()')
-    assert python3_prog != "", python3_err
-    assert python3_prog == sys.executable
+    rv = vim.exec_lua('''
+        local prog, err = vim.provider.python.detect_by_module("neovim")
+        return { prog = prog, err = err }''')
+    assert rv['prog'] != "", rv['err']
+    assert rv['prog'] == sys.executable
 
     assert sys.executable == vim.command_output(
         'python3 import sys; print(sys.executable)')