tools: Make dfu.py work with python3 again.

[dhylands]

Without this change, running:

python3 ../../tools/dfu.py -d build-PYBV11/firmware.dfu

produces the following error:

File: "build-PYBV11/firmware.dfu"
b'DfuSe' v1, image size: 367757, targets: 1
Traceback (most recent call last):
  File "../../tools/dfu.py", line 160, in <module>
    parse(infile, dump_images=options.dump_images)
  File "../../tools/dfu.py", line 42, in parse
    tprefix["name"] = cstring(tprefix["name"])
  File "../../tools/dfu.py", line 23, in cstring
    return string.split("\0", 1)[0]
TypeError: a bytes-like object is required, not 'str'

With the PR applied, the following output is produced:

File: "build-PYBV11/firmware.dfu"
b'DfuSe' v1, image size: 367757, targets: 1
b'Target' 0, alt setting: 0, name: "b'ST...'", size: 367472, elements: 2
  0, address: 0x08000000, size: 14632
    DUMPED IMAGE TO "build-PYBV11/firmware.dfu.target0.image0.bin"
  1, address: 0x08020000, size: 352824
    DUMPED IMAGE TO "build-PYBV11/firmware.dfu.target0.image1.bin"
usb: 0483:df11, device: 0x0000, dfu: 0x011a, b'UFD', 16, 0x1264d2f5

[codecov-commenter]

Codecov Report

Merging #7931 (8815203) into master (7f14344) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #7931   +/-   ##
=======================================
  Coverage   98.24%   98.24%           
=======================================
  Files         154      154           
  Lines       20007    20007           
=======================================
  Hits        19656    19656           
  Misses        351      351           

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[dpgeorge]

Thank you! Merged in cb99ca9