Use a better python problem matcher

The old matcher only worked if the error was raised with `raise Exception('single quotes')`.

This represents a miniscule fraction of errors; for instance, `l[37]` on a short list `l` can raise `IndexError`, and any call to a builtin C function is not going to trace back to a `raise` call.

Instead, this just matches the first line without fail that comes after the context line.

Note that this is still not foolproof; in Python 3.10, `SyntaxError`s are produced as 
```
  File "<stdin>", line 1
    foo(x, z for z in range(10), t, w)
           ^^^^^^^^^^^^^^^^^^^^
SyntaxError: Generator expression must be parenthesized
```

This matcher will incorrectly pick up `           ^^^^^^^^^^^^^^^^^^^^` as the error message, but the previous behavior was to not pick up any error message at all.

As far as I can tell, this is impossible to handle correctly; the grammar of problem matchers is far too limiting.

.github/python.json

@@ -4,13 +4,16 @@
             "owner": "python",
             "pattern": [
                 {
-                    "regexp": "^\\s*File\\s\\\"(.*)\\\",\\sline\\s(\\d+),\\sin\\s(.*)$",
+                    "regexp": "^  File \"([^\"]*)\", line (\\d+).*",
                     "file": 1,
                     "line": 2
                 },
                 {
-                    "regexp": "^\\s*raise\\s(.*)\\(\\'(.*)\\'\\)$",
+                    "regexp": "^    .*$"
-                    "message": 2
+                },
+                {
+                    "regexp": "^(?!  File)(.*)\\s*$",
+                    "message": 1
                 }
             ]
         }