linter: adjust the parsing to accommodate for a modern 'pyflakes'

In version 2.2.0, pyflakes changed its output format,
from 'filename:line: text' to 'filename:line:column text'.

This fixes https://savannah.gnu.org/bugs/?62057.

Problem existed since version 2.9.0, commit 5dcf375f.

(That commit tried to compensate for an introductory message from gcc
that no longer seems to exist.)

src/text.c

@@ -2652,10 +2652,9 @@ void do_linter(void)
 
 				/* The recognized format is "filename:line:column: message",
 				 * where ":column" may be absent or be ",column" instead. */
-				if (strstr(message, ": ") != NULL) {
-					filename = strtok(onelint, ":");
+				if ((filename = strtok(onelint, ":")) != NULL) {
 					if ((linestr = strtok(NULL, ":")) != NULL) {
-						if ((maybecol = strtok(NULL, ":")) != NULL) {
+						if ((maybecol = strtok(NULL, " ")) != NULL) {
 							ssize_t tmplineno = 0, tmpcolno = 0;
 							char *tmplinecol;
 
@@ -2684,7 +2683,7 @@ void do_linter(void)
 							curlint->prev = tmplint;
 							if (curlint->prev != NULL)
 								curlint->prev->next = curlint;
-							curlint->msg = copy_of(strstr(message, ": ") + 2);
+							curlint->msg = copy_of(strstr(message, " ") + 1);
 							curlint->lineno = tmplineno;
 							curlint->colno = tmpcolno;
 							curlint->filename = copy_of(filename);