Make Range instances somewhat immutable

[quicklizard99]

This PR addresses two problems with Range.

1. Attributes of Range instances can be set, bypassing validation and violating class invariants:

   >>> from xocto import ranges
   >>> r = ranges.Range(0, 1)
   >>> r.start = 2
   >>> r.boundaries = '😱'

2. Additional attributes can be set on instances of classes derived from Range

   Range uses __slots__, which prevents creation of instance dictionaries. Classes derived from Range do not have __slots__ = (), which means that instance dictionaries are created:

   >>> from xocto import localtime
   >>> r = ranges.FiniteDatetimeRange(localtime.datetime(2000, 1, 1), localtime.datetime(2000, 1, 3))
   >>> r.some_other_thing = "I shouldn't be here"
   >>> r.__dict__
   {'some_other_thing': "I shouldn't be here"}

Fix these problems:

• add __slots__ = () to classes that have Range as a base class (preventing creation of instance dicts),
• add Range.__setattr__ to raise AttributeError, and
• alter Range.__init__ to use the same object.__setattr__ trick as attrs to set attributes during initialisation.

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: xocto/ranges.py

Function	Unhandled Issue
__init__	ValueError: Invalid boundaries for range /propert... Event Count: 1.0k
__init__	ValueError: Invalid boundaries for range octoener... Event Count: 551
__init__	ValueError: Invalid boundaries for range octoener... Event Count: 164
__init__	ValueError: Invalid boundaries for range octoener... Event Count: 84
__init__	ValueError: Invalid boundaries for range /meter-r... Event Count: 16

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[quicklizard99]

These types were previously implicitly declared when __init__ set attributes.

[quicklizard99]

Setting the default value using a class attribute does not play with __slots__. I have moved this default to __init__.

[quicklizard99]

These exception messages consistent with those raised by collections.namedtuple.

[jamesbeith]

Perhaps explicit is better than implicit, but I wonder if we could delegate this branch to super()?

[quicklizard99]

That is a good point. Change to use super

[jarshwah]

Good change, thanks for making it. As this is a potentially breaking change we'll need to at least bump the minor version when releasing.