tools.checksum: Fix CRC32

The name `data` was being reused for a separate value an causing a crash.

Fixes the error:

    Traceback (most recent call last):
    File "/home/david/work/pybricks/pybricksdev/.venv/bin/pybricksdev", line 5, in <module>
        main()
    File "/home/david/work/pybricks/pybricksdev/pybricksdev/cli/__init__.py", line 413, in main
        asyncio.run(subparsers.choices[args.tool].tool.run(args))
    File "/usr/lib/python3.8/asyncio/runners.py", line 44, in run
        return loop.run_until_complete(main)
    File "/usr/lib/python3.8/asyncio/base_events.py", line 616, in run_until_complete
        return future.result()
    File "/home/david/work/pybricks/pybricksdev/pybricksdev/cli/__init__.py", line 242, in run
        firmware, metadata = await create_firmware(args.firmware, args.name)
    File "/home/david/work/pybricks/pybricksdev/pybricksdev/flash.py", line 102, in create_firmware
        crc32_checksum(io.BytesIO(firmware), metadata["max-firmware-size"] - 4),
    File "/home/david/work/pybricks/pybricksdev/pybricksdev/tools/checksum.py", line 129, in crc32_checksum
        data = int.from_bytes(data[index : index + 4], "little")
    TypeError: 'int' object is not subscriptable

Also add type hints while we are touching this.

Author: dlech

CHANGELOG.md

@@ -4,6 +4,11 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Fixed
+- Fixed crash in CRC32 checksum.
+
 ## [1.0.0-alpha.12]
 
 ### Changed

pybricksdev/tools/checksum.py

@@ -86,11 +86,11 @@ def sum_complement(data: BytesIO, max_size: int) -> int:
 )
 
 
-def _dword(value):
+def _dword(value: int) -> int:
     return value & 0xFFFFFFFF
 
 
-def _crc32_fast(crc, data):
+def _crc32_fast(crc, data: int) -> int:
     crc, data = _dword(crc), _dword(data)
     crc ^= data
     for _ in range(8):
@@ -126,6 +126,6 @@ def crc32_checksum(data: BytesIO, max_size: int) -> int:
 
     crc = 0xFFFFFFFF
     for index in range(0, len(data), 4):
-        data = int.from_bytes(data[index : index + 4], "little")
-        crc = _crc32_fast(crc, data)
+        word = int.from_bytes(data[index : index + 4], "little")
+        crc = _crc32_fast(crc, word)
     return crc