micro opt: callers shall provide a memoryview to .decompress()

if they would call with a bytes object, it would usually cause
1 object copy to remove the compression type bytes.

src/borg/compress.pyx

@@ -90,8 +90,8 @@ cdef class CompressorBase:
 
     def decompress(self, data):
         """
-        Decompress *data* (bytes) and return bytes result. The leading Compressor ID
-        bytes need to be present.
+        Decompress *data* (preferably a memoryview, bytes also acceptable) and return bytes result.
+        The leading Compressor ID bytes need to be present.
 
         Only handles input generated by _this_ Compressor - for a general purpose
         decompression method see *Compressor.decompress*.
@@ -489,8 +489,6 @@ class ObfuscateSize(CompressorBase):
         return super().compress(obfuscated_data)  # add ID header
 
     def decompress(self, data):
-        if not isinstance(data, memoryview):
-            data = memoryview(data)
         obfuscated_data = super().decompress(data)  # remove obfuscator ID header
         compr_size = self.header_fmt.unpack(obfuscated_data[0:self.header_len])[0]
         compressed_data = obfuscated_data[self.header_len:self.header_len+compr_size]

src/borg/crypto/key.py

@@ -355,7 +355,7 @@ class AESKeyBase(KeyBase):
             raise IntegrityError(f"Chunk {bin_to_hex(id)}: Could not decrypt [{str(e)}]")
         if not decompress:
             return payload
-        data = self.decompress(payload)
+        data = self.decompress(memoryview(payload))
         self.assert_id(id, data)
         return data
 
@@ -745,7 +745,7 @@ class AEADKeyBase(KeyBase):
             raise IntegrityError(f"Chunk {bin_to_hex(id)}: Could not decrypt [{str(e)}]")
         if not decompress:
             return payload
-        data = self.decompress(payload)
+        data = self.decompress(memoryview(payload))
         self.assert_id(id, data)
         return data