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.

Added initial tests for FFmpegConcatCommand (ref #31).

Ensured that frame type is ignored in append_concat_filter().

Fixed incorrect default initialisation of punch in/out value.

Cleaned up test documentation (ref #31).

Added subtests to make failures clearer (ref #31).

Added expected filter options to shared tests (ref #31).

Refactored shared testing code into a TestCase superclass (ref #31).

Renamed test case classes (ref #31).

Removed inadvertent maxDiff setting (ref #31).

Added tests for FFmpegCommand class (ref #31).

Updated test options to something more meaningful (ref #31).

Added tests for FFprobeCommand (ref #31).

Removed unnecessary explicit attribute declarations (ref #31).

Fixed code type (ref #31).

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