Regression fix: leave R prefixes capitalization alone

`black.strings.get_string_prefix` used to lowercase the extracted
prefix before returning it. This is wrong because 1) it ignores the
fact we should leave R prefixes alone because of MagicPython, and 2)
there is dedicated prefix casing handling code that fixes issue 1.
`.lower` is too naive.

This was originally fixed in 20.8b0, but was reintroduced since 21.4b0.

CHANGES.md

@@ -11,6 +11,7 @@
 - Restored compatibility with Click 8.0 on Python 3.6 when LANG=C used (#2227)
 - Add extra uvloop install + import support if in python env (#2258)
 - Fix --experimental-string-processing crash when matching parens are not found (#2283)
+- Fix regression where `R` prefixes would be lowercased for docstrings (#2285)
 
 ### _Blackd_
 

src/black/strings.py

@@ -87,7 +87,7 @@ def get_string_prefix(string: str) -> str:
     prefix = ""
     prefix_idx = 0
     while string[prefix_idx] in STRING_PREFIX_CHARS:
-        prefix += string[prefix_idx].lower()
+        prefix += string[prefix_idx]
         prefix_idx += 1
 
     return prefix

tests/data/string_prefixes.py

@@ -6,6 +6,17 @@
 r"hello"
 fR"hello"
 
+
+def docstring_singleline():
+    R"""2020 was one hell of a year. The good news is that we were able to"""
+
+
+def docstring_multiline():
+    R"""
+    clear out all of the issues opened in that time :p
+    """
+
+
 # output
 
 
@@ -16,3 +27,13 @@
 b"hello"
 r"hello"
 fR"hello"
+
+
+def docstring_singleline():
+    R"""2020 was one hell of a year. The good news is that we were able to"""
+
+
+def docstring_multiline():
+    R"""
+    clear out all of the issues opened in that time :p
+    """