Fix normalize_string_prefix not lowercasing uppercase T t-string prefix

CHANGES.md

@@ -27,6 +27,9 @@
   `from x import (  # fmt: skip`) when a standalone comment is among the bracket's
   contents: the whole statement is now preserved instead of being reformatted (and
   previously crashing) (#5161)
+- Fix `normalize_string_prefix` not lowercasing the uppercase `T` prefix of
+  t-strings (e.g. `T"..."` was left unchanged instead of being normalized to
+  `t"..."`) (#XXXX)
 
 ### Preview style
 

src/black/strings.py

@@ -150,6 +150,7 @@ def normalize_string_prefix(s: str) -> str:
     new_prefix = (
         orig_prefix.replace("F", "f")
         .replace("B", "b")
+        .replace("T", "t")
         .replace("U", "")
         .replace("u", "")
     )

tests/data/cases/pep_750.py

@@ -40,6 +40,11 @@
     {1}_cte AS ()'''}
 '''
 
+# Uppercase T prefix is normalized to lowercase t
+x = T"bar {1 + 1}"
+x = T'bar {1 + 1}'
+rT'\{{\}}'
+
 # output
 x = t"foo"
 x = t"foo {{ {2 + 2}bar {{ baz"
@@ -81,3 +86,8 @@
     WITH {f'''
     {1}_cte AS ()'''}
 """
+
+# Uppercase T prefix is normalized to lowercase t
+x = t"bar {1 + 1}"
+x = t"bar {1 + 1}"
+rt"\{{\}}"