Removed Pipe slots implementation.

Pipe inherits multiply and previously looked like this:

    class Pipe(abctools.AbjadObject, subprocess.Popen):

        ...

        __slots__ (
            '_arguments'
            '_executable',
            '_timeout',
        )

This caused Pipe to break copy.

Removing multiple inheritance (and preserving slots) did fix the class's copy
functionality:

    class Pipe(subprocess.Popen):

        ...

        __slots__ (
            '_arguments'
            '_executable',
            '_timeout',
        )

Preserving multiple inheritance (but removing slots) also fixed the class's
copy functionality:

    class Pipe(abctools.AbjadObject, subprocess.Popen):

        ...

Made the decision to preserve multiple inheritance but remove slots.

There's something about multiple inheritance breaking slots in the general
case.

Perhaps the breaking of copy functionality is another version of this.

At any rate, Pipe now copies correctly.

    >>> pipe = documentationtools.Pipe()
    >>> pipe
    Pipe(executable='python', arguments=('-i',), timeout=0)

    >>> import copy
    >>> copy.copy(pipe)
    Pipe(executable='python', arguments=('-i',), timeout=0)