Rebuilt for https://fedoraproject.org/wiki/Fedora_38_Mass_Rebuild

Retry if shallow clone fails

Shallow git clone if no reference is used
Select by the source of object
Enable the flake8 check, fix detected issues
Make when in adjust optional, add true|false expr.
Version as fmf flag and module attribute

Implement a directive for disabling inheritance
Add support for prepending items during merging
Explicitly convert context value to string
Workaround s390x ruamel read issue
Support reference schemas in validation function
Add JSON Schema validation method
Fixes blocking with symbolic link loops
Use a better way to detect the default git branch
Add unit tests for comma operator in context rules
Add examples demonstrating the context syntax

Migrate yaml parsing to ruamel.yaml
Drop support for Python 2
Add extra requires and enable readthedocs config (#143)
Filter accept value with a colon
Adjust style to fix issues revealed by pre-commit
Enable pre-commit, update contribution docs
Disable integration testing with tmt for centos-7

Add support for empty files and virtual nodes
Change outcome of CannotDecide operations
Enable copr builds from master for epel-7 as well
Define dict method to get child or data item
Keep original adjust rules when adjusting nodes
Use a deep copy when storing raw node data
Fix warnings in pytest

Do not use mutable objects as default arguments
A bunch of adjustments for the epel-7 build

Implement the new 'fmf clean' command
Functions to manipulate cache
Fetch the whole tree using utils.fetch_tree()
Raise FileException on duplicate YAML keys
Allow to set desired cache location
Adjust support for storing modified data to disk
Add support for storing modified metadata to disk
Prevent exploring tests in the tmt directory
Simplify special distro comparison examples
Store command output in run(), fetch() exceptions
Use filelock for locking, expand test coverage
Adjust locking for Tree.node() and utils.fetch()
Use locking to checkout/read tree
Enable integration testing with tmt, some cleanup
Implement a new method copy() for cloning nodes
Adjust fetch() with custom destination and env
Support calls fetch(destination, env) and run(env)
Use the origin default branch instead of master
Store commit hash if the tree is under a git repo