Fix invalid E231 error with f-strings (#8369)

## Summary

We were considering the `{` within an f-string to be a left brace, which
caused the "space-after-colon" rule to trigger incorrectly.

Closes https://github.com/astral-sh/ruff/issues/8299.

crates/ruff_linter/resources/test/fixtures/pycodestyle/E23.py

@@ -40,5 +40,11 @@ f"{(a:=1)}"
 f"{(lambda x:x)}"
 f"normal{f"{a:.3f}"}normal"
 
+#: Okay
+snapshot.file_uri[len(f's3://{self.s3_bucket_name}/'):]
+
+#: E231
+{len(f's3://{self.s3_bucket_name}/'):1}
+
 #: Okay
 a = (1,

crates/ruff_linter/src/rules/pycodestyle/rules/logical_lines/missing_whitespace.rs

@@ -64,14 +64,14 @@ pub(crate) fn missing_whitespace(line: &LogicalLine, context: &mut LogicalLinesC
         match kind {
             TokenKind::FStringStart => fstrings += 1,
             TokenKind::FStringEnd => fstrings = fstrings.saturating_sub(1),
-            TokenKind::Lsqb => {
+            TokenKind::Lsqb if fstrings == 0 => {
                 open_parentheses = open_parentheses.saturating_add(1);
                 prev_lsqb = token.start();
             }
-            TokenKind::Rsqb => {
+            TokenKind::Rsqb if fstrings == 0 => {
                 open_parentheses = open_parentheses.saturating_sub(1);
             }
-            TokenKind::Lbrace => {
+            TokenKind::Lbrace if fstrings == 0 => {
                 prev_lbrace = token.start();
             }
             TokenKind::Colon if fstrings > 0 => {

crates/ruff_linter/src/rules/pycodestyle/snapshots/ruff_linter__rules__pycodestyle__tests__E231_E23.py.snap

@@ -121,4 +121,24 @@ E23.py:33:6: E231 [*] Missing whitespace after ','
 35 35 | # Okay because it's hard to differentiate between the usages of a colon in a f-string
 36 36 | f"{a:=1}"
 
+E23.py:47:37: E231 [*] Missing whitespace after ':'
+   |
+46 | #: E231
+47 | {len(f's3://{self.s3_bucket_name}/'):1}
+   |                                     ^ E231
+48 | 
+49 | #: Okay
+   |
+   = help: Add missing whitespace
+
+ℹ Fix
+44 44 | snapshot.file_uri[len(f's3://{self.s3_bucket_name}/'):]
+45 45 | 
+46 46 | #: E231
+47    |-{len(f's3://{self.s3_bucket_name}/'):1}
+   47 |+{len(f's3://{self.s3_bucket_name}/'): 1}
+48 48 | 
+49 49 | #: Okay
+50 50 | a = (1,
+