patch 8.1.0743: giving error messages is not flexible

Problem:    Giving error messages is not flexible.
Solution:   Add semsg().  Change argument from "char_u *" to "char *", also
            for msg() and get rid of most MSG macros. (Ozaki Kiichi, closes
            #3302)  Also make emsg() accept a "char *" argument.  Get rid of
            an enormous number of type casts.
diff --git a/src/if_python3.c b/src/if_python3.c
index a41d6ac..4bbbdd1 100644
--- a/src/if_python3.c
+++ b/src/if_python3.c
@@ -634,7 +634,7 @@
     if (python_loaded())
     {
 	if (verbose)
-	    EMSG(_("E837: This Vim cannot execute :py3 after using :python"));
+	    emsg(_("E837: This Vim cannot execute :py3 after using :python"));
 	return FAIL;
     }
 # endif
@@ -646,7 +646,7 @@
     if (!hinstPy3)
     {
 	if (verbose)
-	    EMSG2(_(e_loadlib), libname);
+	    semsg(_(e_loadlib), libname);
 	return FAIL;
     }
 
@@ -658,7 +658,7 @@
 	    close_dll(hinstPy3);
 	    hinstPy3 = 0;
 	    if (verbose)
-		EMSG2(_(e_loadfunc), py3_funcname_table[i].name);
+		semsg(_(e_loadfunc), py3_funcname_table[i].name);
 	    return FAIL;
 	}
     }
@@ -693,7 +693,7 @@
 	close_dll(hinstPy3);
 	hinstPy3 = 0;
 	if (verbose)
-	    EMSG2(_(e_loadfunc), "PyUnicode_UCSX_*");
+	    semsg(_(e_loadfunc), "PyUnicode_UCSX_*");
 	return FAIL;
     }
 
@@ -864,7 +864,7 @@
 #ifdef DYNAMIC_PYTHON3
 	if (!python3_enabled(TRUE))
 	{
-	    EMSG(_("E263: Sorry, this command is disabled, the Python library could not be loaded."));
+	    emsg(_("E263: Sorry, this command is disabled, the Python library could not be loaded."));
 	    goto fail;
 	}
 #endif