Htimeseries

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- When saving in file format, the Timezone= parameter now merely
contains +HHmm.
- The HTimeseries.timezone attribute has been abolished; use
HTimeseries.data.index.tz instead.
- When initialized with a dataframe, it checks that the index is aware.

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- Fixed error with unspecified time zone in HTimeseries objects created
empty. Now creating an empty object with ``HTimeseries()`` assumes
``default_tzinfo=ZoneInfo("UTC")``.

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- Fixed crash when reading csv with aware timestamps.

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- Fixed crash when reading csv with only a date column.

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- Python 3.9 is now required (the reason for this is that
backports.zoneinfo is behaving differently from zoneinfo).
- Only pandas>=1.5,<2 is now supported. Pandas<1.5 probably did not
work in 5.0.0 either, but it had not been discovered. Pandas>=2
handles ambiguous times differently and is therefore still
unsupported.
- Increased CSV reading speed by two orders of magnitude.

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- Removed compatibility with pandas<1. pandas 1 and 2 are now supported.
This helps avoid an error when reading a data file spanning a time
interval that contains a change to or from DST.