tests: check for bzr support by importing bzrlib This is better than starting bzr --version, because it works correctly when running tests with a non-default Python (version).

convert: fixed python2.3 incompatibility in bzr source (generator expression)

Improve error handling in osutil.c 1) In posix part set error when path is too long so instead of SystemError: error returned without exception set it will raise ValueError: path too long 2) In Win32 part replace generic PyErr_SetExcFromWindowsErrWithFilename by PyErr_SetFromWindowsErrWithFilename The exception returned is WinError(based on OSError) and some rudimentary errno translation is performed from Windows error range to errno module friendly range so errors like ENOENT can be handled via symbolic constant and consistently between Win32 and Posix.

`source` doesn't work for some /bin/sh, use `.` instead