Pyvcd

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* Add python_requires >=3.6 to setup
* Packaging changes related to PEP-517

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* Breaking changes:

* Python 3.6 is minimum version; drop Python 2 support
* Remove ident argument from VCDWriter.register_var()

* Deprecations:

* Enums for scope, variable, and timescale types
* Enums for GTKWave flags and colors

* Features:

* Inline type annotations, checkable with Mypy
* Use base-94 encoding for variable identifiers
* Improved performance

* Repairs:

* Repair default string variable value
* Ensure compound vector value correctness

* Development environment changes:

* Add top-level Makefile with targets for common commands
* Format code using black
* Format imports using isort
* Check type annotations with Mypy
* Use GitHub Actions for CI; drop Travis

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* Repair event variable changes (14)

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* Repair mis-formatted variable identifiers in dumps
* Exclude event and string types from dump_off
* Avoid duplicate timestamps in VCD output
* Avoid duplicate values in VCD output
* Improve performance when registering many variables in a scope (12)

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* Improve runtime performance by using write() (9)
* Update package classifiers to note Python 3.8 support

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* Add "string" variable type
* Repair deprecated import of ABC's from collections.abc