requires/types: introduce expression type (expr.py)

[mustafakemalgilor]

expression is a new requirement type that allows users to express a requirement in human-readable, SQL-like syntax. It has proper grammar to allow users to write checks in as easily digestible manner, which will improve the QOL for scenario writers and code reviewers. The grammar is constructed using the pyparsing library.

Example:

checks:
  not_using_iommu_passthrough_for_suitable_amd: |
    (@hotsos.core.plugins.kernel.sysfs.CPU.vendor == 'authenticamd') AND
    (@hotsos.core.plugins.system.SystemBase.virtualisation_type == None) AND
    NOT('iommu=pt' IN @hotsos.core.plugins.kernel.KernelBase.boot_parameters)    

The current grammar supports the following constructs:

• Keywords (True, False, None)
• Constants (Integer, Float, String literal)
• Runtime variables (Python properties)
• Functions(len, not, file, systemd, read_ini, read_cert)
• Arithmetic operators(sign, mul, div, add, sub, exp)
• Comparison operators(<, <=, >, >=, ==, !=, in)
• Logical operators(and, or, not)
• Comments('#', '//', '/.../')

Also, the following changes have been made:

• Updated requirement_types.rst to document the new Expression requirement type.
• Moved property resolver logic from YPropertyBase to PythonEntityResolver class.
• Added pyparsing as a dependency.
• Added unit tests for the new code.

This patch rewrites some of the scenario checks in the new expression syntax to demonstrate the difference and provide examples.

[mustafakemalgilor]

IDK why the "Test / test (3.8, ubuntu-20.04) (pull_request)" job is failing, all tests are green in my env. I'll re-run them in a py3.8 environment and get to the bottom of it.

[mustafakemalgilor]

@dosaboy fixed CI issues, ready for review. please feel free to ping me should you need to ask anything.

[mustafakemalgilor]

This feature is intended to be used in conjunction with the aliasing feature: https://github.com/canonical/hotsos/pull/929