Remove boost.python.injector

[Anthchirp]

This is known to not work in Python 3 and its continued availability makes debugging harder than necessary.

Any places still refering to boost.python.injector should use the @boost.python.inject_into decorator instead.

Merging this will break code that still uses the old method - that is by design.

~Broken places can be fixed following the instructions given in the file~ Affected places can be updated by changing the code from eg.

class CrystalExt(boost.python.injector, Crystal):

to

@boost.python.inject_into(Crystal)
class _(object):

[Anthchirp]

Although nobody commented on this I will change the approach here.

There is a nicer way of achieving the same aim by introducing DeprecationWarnings first. This will make it easier for developers relying on that functionality. I will commit that to master shortly, then update this pull request, and then we can remove the function at some point in the future.