this commit deliberately *duplicates* the dependency information between
the setup.py and requirements.pip.

The rationale behind it is that one of them is the generic dependency
information, to be passed to system helpers in order to package it.

The requirements.pip has any pinning information that we want to impose
during development, and it behaves as a well-known set.

Read https://caremad.io/posts/2013/07/setup-vs-requirement/ for more
explanations.

- Resolves: #8830

At this point tests are not yet collectable
in the new environment. But no regressions observed.

Our magic value wasn't being used and were represented as a string.
Refactored it to a constant, increased it's size to 2 bytes and optimzed
is_symmetrically_encrypted to look for the magic and symmetrically
encrypted flag under base64 encoding. Most file types will use this
feature to help identifying themselves, so it got refactored to serve
the purpose it was created.

Those are already present, but we are using the ones coming from our
dependencies. Explicit is better than implicit.

modifying original PR [0] by cristoph to account for the recent
vendoring of l2db code, which means we no longer depend on u1db/dirspec.

I expect the whole mess about the venv setup to be further simplified
pretty soon, since we are going to merge most of the leap.* packages
into a couple of repos.

[0] https://github.com/leapcode/soledad/pull/327