Issue #15054: Fix incorrect tokenization of 'b' and 'br' string literals.

Patch by Serhiy Storchaka.
diff --git a/Lib/test/test_tokenize.py b/Lib/test/test_tokenize.py
index 38da106..a51e781 100644
--- a/Lib/test/test_tokenize.py
+++ b/Lib/test/test_tokenize.py
@@ -278,6 +278,31 @@
     OP         '+'           (1, 32) (1, 33)
     STRING     'UR"ABC"'     (1, 34) (1, 41)
 
+    >>> dump_tokens("b'abc' + B'abc'")
+    STRING     "b'abc'"      (1, 0) (1, 6)
+    OP         '+'           (1, 7) (1, 8)
+    STRING     "B'abc'"      (1, 9) (1, 15)
+    >>> dump_tokens('b"abc" + B"abc"')
+    STRING     'b"abc"'      (1, 0) (1, 6)
+    OP         '+'           (1, 7) (1, 8)
+    STRING     'B"abc"'      (1, 9) (1, 15)
+    >>> dump_tokens("br'abc' + bR'abc' + Br'abc' + BR'abc'")
+    STRING     "br'abc'"     (1, 0) (1, 7)
+    OP         '+'           (1, 8) (1, 9)
+    STRING     "bR'abc'"     (1, 10) (1, 17)
+    OP         '+'           (1, 18) (1, 19)
+    STRING     "Br'abc'"     (1, 20) (1, 27)
+    OP         '+'           (1, 28) (1, 29)
+    STRING     "BR'abc'"     (1, 30) (1, 37)
+    >>> dump_tokens('br"abc" + bR"abc" + Br"abc" + BR"abc"')
+    STRING     'br"abc"'     (1, 0) (1, 7)
+    OP         '+'           (1, 8) (1, 9)
+    STRING     'bR"abc"'     (1, 10) (1, 17)
+    OP         '+'           (1, 18) (1, 19)
+    STRING     'Br"abc"'     (1, 20) (1, 27)
+    OP         '+'           (1, 28) (1, 29)
+    STRING     'BR"abc"'     (1, 30) (1, 37)
+
 Operators
 
     >>> dump_tokens("def d22(a, b, c=2, d=2, *k): pass")