Python: Fix syntax error when = is used as a format fill character

An example (provided by @redsun82) is the string `f"{x:=^20}"`. Parsing
this (with unnamed nodes shown) illustrates the problem:

```
module [0, 0] - [2, 0]
  expression_statement [0, 0] - [0, 11]
    string [0, 0] - [0, 11]
      string_start [0, 0] - [0, 2]
      interpolation [0, 2] - [0, 10]
        "{" [0, 2] - [0, 3]
        expression: named_expression [0, 3] - [0, 9]
          name: identifier [0, 3] - [0, 4]
          ":=" [0, 4] - [0, 6]
          ERROR [0, 6] - [0, 7]
            "^" [0, 6] - [0, 7]
          value: integer [0, 7] - [0, 9]
        "}" [0, 9] - [0, 10]
      string_end [0, 10] - [0, 11]
```
Observe that we've managed to combine the format specifier token `:` and
the fill character `=` in a single token (which doesn't match the `:` we
expect in the grammar rule), and hence we get a syntax error.

If we change the `=` to some other character (e.g. a `-`), we instead
get

```
module [0, 0] - [2, 0]
  expression_statement [0, 0] - [0, 11]
    string [0, 0] - [0, 11]
      string_start [0, 0] - [0, 2]
      interpolation [0, 2] - [0, 10]
        "{" [0, 2] - [0, 3]
        expression: identifier [0, 3] - [0, 4]
        format_specifier: format_specifier [0, 4] - [0, 9]
          ":" [0, 4] - [0, 5]
        "}" [0, 9] - [0, 10]
      string_end [0, 10] - [0, 11]
```
and in particular no syntax error.

To fix this, we want to ensure that the `:` is lexed on its own, and the
`token(prec(1, ...))` construction can be used to do exactly this.

Finally, you may wonder why `=` is special here. I think what's going on
is that the lexer knows that `:=` is a token on its own (because it's
used in the walrus operator), and so it greedily consumes the following
`=` with this in mind.

Author: tausbn

python/extractor/tsg-python/tsp/grammar.js

@@ -1168,7 +1168,7 @@ module.exports = grammar({
     _not_escape_sequence: $ => token.immediate('\\'),
 
     format_specifier: $ => seq(
-      ':',
+      token(prec(1,':')),
       repeat(choice(
         token(prec(1, /[^{}\n]+/)),
         alias($.interpolation, $.format_expression)