Oscfs

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- use Python3 by default now.
- various small bugfixes:
- fix error if a binary build artifact has zero file size
- fix symlink target in package update links (symlink to maintenance
incidents).
- new --no-urlopen-wrapper command line option to disable the HTTP connection
reuse hack. This is helpful when working against build.suse.de, where
two-factor-authentication is now required and the hack doesn't work with
that anymore.
- new --no-bin-cache command line option to disable caching of large binary files.
- new --use-logfile command line option to write errors and debugging
information to file. This is helpful when oscfs is running as a daemon and
stdout/stderr are discarded.
- implements transparent retry if the OBS server responds with HTTP status 503
(service unavailable).

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- fix minor error in urlopen wrapper occuring in some Python versions,
causing an exception upon start.

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- fix error if a package filename contained the '' character.
- fix authentication error detection during startup.
- enable urlopenwrapper hack again which speeds up consecutive requests
drastically.
- source code ported to Python3 only, removed all Python2 compatibility.
Source code is now conforming largely to PIP and flake8 standards.

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- fix error when reading from some "buildlog" nodes that contained non-utf8
encoding data.
- fix problem with grep on larger files, because more data was returned by
read() than actually requested when unicode data was involved.

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- fix read() with offset especially with large files. This fixes use of
various tools when applied on oscfs files e.g. `rpm2cpio` or
`tail *.spec` previously failed or behaved strangely.
- be robust against non-ASCII characters in OBS buildlogs.
- apply caching also to actual file content to be more efficient when e.g.
operating on larger files.

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- Added support for running in Python3 with the Python3 osc module installed.
- support mounting of oscfs in non-empty directories.
- minor bugfixes with accessing package and project meta data via `.oscfs`.