Documented how to run unit tests.

Removed unnecessary main body blocks and #! lines (closes #31 for now).

Added unit tests for the FrameSegment class (ref #31).

Added missing test message (ref #31).

Rearranged tests into more appropriate locations (ref #31).

Rationalised some unnecessary code (ref #31).

Added tests for VideoSegment class (ref #31).

Extracted some Segment-specific tests (ref #31).

Added tests for the AudioSegment class (ref #31).

Added shared tests for segment classes (ref #31).

Renamed format variable more sensibly.

Corrected suffix specification for Pathlib (ref #32).

Moved progress bar tests into correct location (WTF?!) (ref #31).

Moved shared tests into package declaration (ref #31).

Added unit tests for ProgressBar class (ref #31).

Added test subpackages for progress bar and segment (ref #31).

Refactored progress bar class into subpackage.

Refactored segment classes into a subpackage.

Further refactored shell command tests into a subpackage.

Refactored shell commands and tests into a subpackage.

Added test for build_complex_filter() (ref #31).

Finished test for append_concat_filter() (ref #31).

Clarified confusing comment.

Added requirement for Python ≥ 3.7 (ref #32).

Dropped use of OrderedDicts (ref #32).

Reduced punch out timedeltas from 20 days to 20 seconds.

Updated superclass initialisation for Python 3 (ref #32).

Made more effective use of subtests (ref #31).

Refactored “type” to “frame_type” in append_concat_filter().

Found some more incorrect timedelta initialisations.