pcf: Remove use of f-string, not compatible with 5.x mpy-cross

Author: jepler

adafruit_bitmap_font/pcf.py

@@ -123,7 +123,7 @@ def __init__(self, f, bitmap_class):
 
         bitmap_format = self.tables[_PCF_BITMAPS].format
         if bitmap_format != 0xE:
-            raise NotImplementedError(f"Unsupported format_ {bitmap_format:x}")
+            raise NotImplementedError("Unsupported format %s" % bitmap_format)
 
         self._accel = self._read_accelerator_tables()
         self._encoding = self._read_encoding_table()