Fix for python3.5 compatibility

rpg/core.py

@@ -227,7 +227,7 @@ def next_read(file, offset_start, offset_end):
 
     # Open the file, GZIP or not
     with (gzip.open(file, "rb") if magic == b"\x1f\x8b"
-          else open(file, "rb")) as in_file:
+          else open(str(file), "rb")) as in_file:
         first_line = in_file.readline().decode('utf-8')
         # FASTQ file
         if first_line.startswith("@"):