file_table_reader/qgis_bootstrap.py

import os
import sys

# QGIS environment
os.environ["QGIS_PREFIX_PATH"] = "/usr"
os.environ["QGIS_HOME"] = "/usr"

os.environ["PROJ_LIB"] = "/usr/share/proj"
os.environ["GDAL_DATA"] = "/usr/share/gdal"
os.environ["QT_PLUGIN_PATH"] = "/usr/lib/x86_64-linux-gnu/qt5/plugins"

os.environ["QT_QPA_PLATFORM"] = "offscreen"

# QGIS Python plugins (THIS IS THE MISSING PART)
sys.path.append("/usr/share/qgis/python")
sys.path.append("/usr/share/qgis/python/plugins")

# Python modules (from system)
sys.path.append("/usr/lib/python3/dist-packages")
sys.path.append("/usr/lib/python3/dist-packages/qgis")


from qgis.core import QgsApplication, QgsProviderRegistry
from qgis.analysis import QgsNativeAlgorithms
import processing
from processing.core.Processing import Processing

def start_qgis():
    qgs = QgsApplication([], False)
    qgs.initQgis()
    Processing.initialize()
    QgsProviderRegistry.instance()
    qgs.processingRegistry().addProvider(QgsNativeAlgorithms())
    return qgs