Stix2

* 513 Fixes check for reference properties to apply to all custom objects, not
just custom observables (maybe-sybr)
* 526 Fixes TAXIICollection queries to propagate HTTPErrors, not ignore them
* 528 Fixes bug where all TAXII filters were converted to exact matches

This version aligns with the STIX 2.1 CS 03 (WD12) document, also known as the "OASIS Standard".

Main Changes
509 Add Sighting.summary default value
503 Revamp how STIX content customization is detected and enforced enhancement
500 fp write for STIX Objects
492 Drop 'six' dependency (backwards breaking)
490 Change canonicalization to normalization
489 Fix observation expression DNF transformer to preserve FOLLOWEDBY order.
485 Update CustomObservable decorator
483 add is_sdo() et al functions
481 Refactor stix2.parsing into more focused modules
480 Support Pagination in TAXII DataStore
476 FileSystemsSink.add creates an additional level of directory hierarchy
458 Versioning refinements
468 Extensions Support

Changes for CS03:
517 Update links in code to current specification
515 Add new infrastructure-type-ov entries
514 Update Network Traffic for CS03
504 Fix sighting.last_seen check

Semantic Similarity
499 Update incident weights for semantic similarity
496 Similarity/Equivalence Changes
493 Update the semantic equivalence user guide page
491 Graph Equivalence Changes

Testing, etc
482 Migrate test to GitHub Actions, update test & check configuration

* 337 Switches fuzzywuzzy dependency for rapidfuzz (maxbachmann)
* 430 Adds ability to mix single objects and lists in the Bundle constructor
* 445, 475 Adds ability to calculate semantic equivalence of indicator patterns
* 449 Adds ability to calculate semantic equivalence of entire graphs of objects
* 427 Fixes protocol_family property on network socket extension
* 436 Fixes pattern visitor to handle expressions with both AND and OR
* 431 Fixes bug when adding custom object to FileSystemSink if the object type
hasn't been registered
* 439 Fixes bug with custom wrapped classes not retaining their name (maybe-sybr)
* 438 Fixes bug with patterns when the object path contains numeric index steps
* 454 Fixes stix2.patterns.make_constant() to create TimestampConstants
* 455 Fixes bug with AND comparisons in patterns
* 460 Fixes bug when retrieving custom object from TAXIICollectionSource if
the object type hasn't been registered
* 444 Fixes bug so CompositeDataSource and deduplicate() handle unversioned
objects correctly
* 467 Fixes bug in semantic equivalence when Location objects don't have
latitude and longitude properties
* 470 Fixes bug where Sighting's where_sighted_refs property couldn't point to
a Location object
* 473 Fixes typo in name of X509V3ExtensionsType class
* 474 Fixes order of object properties when serialized to match examples from
the STIX specification

* 423 Fixes issue with six dependency.

* Fixes issue with PyPI.

* 418 Drops support for Python versions older than 3.5
* 397 Drops python-dateutil dependency to just use built-in datetime instead
* 392, 402 Fixes bugs with deterministic ID generation
* 393 Fixes bug when using SSDEEP hashes
* 404 Fixes bug with "import *"
* 400 Fixes bug with pattern builder and STIX 2.0 Start/Stop timestamps
* 407 Fixes bug to allow granular markings with 'id' as selector
* 409 Fixes bug with 2.1 object reference properties
* 401 Fixes bug where some objects can be versioned and revoked that shouldn't
* 417 Improves efficiency of ListProperty