Add constants.py and its test

Task: MIL-001#3

src/weather/__init__.py

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+## @package weather
+#  @brief Reading CSV weather data, from Udemy's 100 Days of Code (day 25).
+#
+#  @mainpage Weather Data
+#
+#  The day-25 lectures read the weather data of one week from the file
+#  weather_data.csv: first with Python's built-in csv library, then with the
+#  pandas library, which makes the same work a one-liner. The package keeps the
+#  two ways side by side: csv_reader.py uses the standard library only, and
+#  analysis.py and export.py need the optional pandas extra.
+#
+#  The constants of the package are in constants.py.
+
+"""Reading CSV weather data, from Udemy's 100 Days of Code (day 25)."""

src/weather/constants.py

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+## @file constants.py
+#  @brief Constants of the weather data project.
+#
+#  Every value that the day-25 lectures fix lives here, so that no other module
+#  contains a magic string or number: the name of the data file, the names of
+#  its columns, the factors of the Fahrenheit conversion and the name of the
+#  exported file.
+
+"""Constants of the weather data project, as fixed by the day-25 lectures."""
+
+from pathlib import Path
+from typing import Final
+
+## @brief Folder of this package; the data file lies next to the modules.
+PACKAGE_DIR: Final[Path] = Path(__file__).resolve().parent
+
+## @brief Name of the weather data file of the lecture.
+DATA_FILE_NAME: Final[str] = "weather_data.csv"
+
+## @brief Full path of the weather data file, independent of the working folder.
+DATA_FILE: Final[Path] = PACKAGE_DIR / DATA_FILE_NAME
+
+## @brief Name of the file that the exported table is written to by default.
+EXPORT_FILE_NAME: Final[str] = "new_data.csv"
+
+## @brief Encoding of the CSV files that are read and written.
+FILE_ENCODING: Final[str] = "utf-8"
+
+## @brief Name of the column that holds the day of the week.
+COLUMN_DAY: Final[str] = "day"
+
+## @brief Name of the column that holds the temperature in degrees Celsius.
+COLUMN_TEMP: Final[str] = "temp"
+
+## @brief Name of the column that holds the weather condition, such as `Sunny`.
+COLUMN_CONDITION: Final[str] = "condition"
+
+## @brief Position of the temperature in a row that `csv.reader` returns.
+TEMP_COLUMN_INDEX: Final[int] = 1
+
+## @brief Numerator of the Celsius to Fahrenheit factor: `temp * 9 / 5 + 32`.
+FAHRENHEIT_NUMERATOR: Final[int] = 9
+
+## @brief Denominator of the Celsius to Fahrenheit factor: `temp * 9 / 5 + 32`.
+FAHRENHEIT_DENOMINATOR: Final[int] = 5
+
+## @brief Offset of the Celsius to Fahrenheit conversion: `temp * 9 / 5 + 32`.
+FAHRENHEIT_OFFSET: Final[int] = 32