Tagging 2.8.2

This (deprecated) property failed when used on a `Spec` item based on a
single-term range (e.g. `==0.1.2` `<2.0.0`).

Closes #82.

Signed-off-by: Raphaël Barrois <raphael.barrois@polytechnique.org>

This attribute wasn't meant to be removed.

Streamline the release process.

Always include every dev/doc/test related files.

Write clear, explicit rules for the `SimpleSpec` syntax.

Their comparison semantics were ill-defined, and mostly an
implementation detail for the old, 'native' specs.

- Use proper theme in development
- Use ReadTheDocs version numbers in titles

This will be used in `sort(..., key=lambda v: v.precedence_key)`.
Remove previous comparison/ordering implementation.

The current implementation relies on a 4-tuple:
- major, minor, patch (as integers) - natural order matches precedence
  rules
- a tuple of identifiers for the prerelease component.

The identifiers for the prerelease components are based on:
- A `NumericIdentifier` class, that compares number using their natural
  order, and always compares lower than non-numeric identifiers
- A `AlphaIdentifier` class for non-numeric identifiers; it compares
  versions using ASCII ordering.
- A `MaxIdentifier` class, that compares higher to any other identifier;
  used to ensure that a non-prerelease version is greater than any of
  its prereleases.

Using `make update` is closer to common patterns than `make
install-deps`; it is also suitable to update the local development
environment.

The internal features from those classes will be removed in future
versions:

- The `Spec` class is incompatible with the support of multiple syntaxes
- The `SpecItem` class was an implementation detail, but doesn't support
  complex `Range` combinations.

The code follows closely the specification available at
https://docs.npmjs.com/misc/semver.html.

Despite similarities, the matching logic is fully separate from the
`native` code, since both might evolve at their own scales.

Instead of choosing the comparison on each `.match()` call, the
expression is converted to a combination of `Range()` expressions
(simple comparison to a semver-compliant `Version`).

`Range()` objects can be combined with `And` and `Or` through the
`AnyOf` and `AllOf` clauses (sticking to Python's naming scheme).

Some specific flags have been provided to those range, allowing users to
subtly alter the matching behaviour - thus accomodating different
versioning schemes:
- `<0.1.2` won't match `0.1.2-rc1`, unless the prerelease_policy flag is
  set to either `always` or `same-patch`
- `<0.1.2` will match `0.1.1-rc1`, unless the `prerelease_policy` flag
  is set to `same-patch`
- `==0.1.2` will always match `0.1.2+build44`, unless the `build_policy`
  is set to `strict`.

The `Spec` item has been updated, alongside `SpecItem`.
Those objects keep the original expression as attributes, but don't use
them for comparisons.

According to the stated goal of "intuitive" behaviour, we want:

``Version('0.1.1-a1') not in Spec('<0.1.1')``.

Tests, code and docs have been fixed.

This simplifies computing neighbouring versions.

This provides more helpful error messages when a test fails.