nvenc fails to load on centos 7

[Antoine Martin]

cannot load nvenc (nvenc encoder): Encoder missing from xpra.codecs.nvenc.encoder
Traceback (most recent call last):
  File "/usr/src/rpmbuild/BUILDROOT/xpra-2.5-0.20181023r20783.el7_5.x86_64/usr/lib64/python2.7/site-packages/xpra/codecs/loader.py", line 53, in codec_import_check
    ic =  __import__(class_module, {}, {}, classname)
  File "xpra/codecs/nvenc/encoder.pyx", line 1152, in init xpra.codecs.nvenc.encoder
  File "xpra/codecs/nvenc/encoder.pyx", line 1149, in xpra.codecs.nvenc.encoder.test_parse
  File "xpra/codecs/nvenc/encoder.pyx", line 1127, in xpra.codecs.nvenc.encoder.c_parseguid
TypeError: must be char, not unicode

[Antoine Martin]

Seems to work OK as of:

• r20805: use byte strings everywhere (no matter what python / cython version is used)
• r20806: chars vs uint8_t
• r20808: don't use "array" module at all

[Antoine Martin]

Backport to v2.4 in r20810.