refactor(database): Let the ImportError exception propagate

Instead of catching the exception during import of the database driver,
change the behavior to let the `ImportError` exception propagate. This
allow to lazy-import the drivers and gracefully handle the missing
dependencies during the configuration.

Author: gfieni

src/powerapi/database/__init__.py

@@ -30,10 +30,3 @@
 from powerapi.database.driver import ReadableDatabase, WritableDatabase, ReadableWritableDatabase, DatabaseDriver
 from powerapi.database.codec import CodecOptions, ReportEncoder, ReportEncoderRegistry, ReportDecoder, ReportDecoderRegistry
 from powerapi.database.exceptions import DBError, ConnectionFailed, NotConnected, WriteFailed, ReadFailed
-from powerapi.database.csv import CSVInput, CSVOutput
-from powerapi.database.mongodb import MongodbInput, MongodbOutput
-from powerapi.database.opentsdb import OpenTSDB
-from powerapi.database.influxdb2 import InfluxDB2
-from powerapi.database.prometheus import Prometheus
-from powerapi.database.socket import Socket
-from powerapi.database.json import JsonInput, JsonOutput

src/powerapi/database/mongodb/driver.py

@@ -29,18 +29,14 @@
 
 from collections.abc import Iterable
 
+from pymongo import MongoClient
+from pymongo.errors import PyMongoError
+
 from powerapi.database.driver import DatabaseDriver, ReadableDatabase, WritableDatabase
 from powerapi.database.exceptions import ConnectionFailed, ReadFailed, WriteFailed
 from powerapi.database.mongodb.codecs import ReportEncoders, ReportDecoders
 from powerapi.report import Report
 
-try:
-    from pymongo import MongoClient
-    from pymongo.errors import PyMongoError
-except ImportError:
-    MongoClient = None
-    PyMongoError = None
-
 
 class _MongodbDriver(DatabaseDriver):
     """

src/powerapi/database/opentsdb/driver.py

@@ -29,17 +29,13 @@
 
 from collections.abc import Iterable
 
+from opentsdb import TSDBClient, TSDBClientException
+
 from powerapi.database.driver import WritableDatabase
 from powerapi.database.exceptions import ConnectionFailed, WriteFailed
 from powerapi.database.opentsdb.codecs import ReportEncoders
 from powerapi.report import Report
 
-try:
-    from opentsdb import TSDBClient, TSDBClientException
-except ImportError:
-    TSDBClient = None
-    TSDBClientException = None
-
 
 class OpenTSDB(WritableDatabase):
     """

src/powerapi/database/prometheus/driver.py

@@ -29,18 +29,14 @@
 
 from collections.abc import Iterable
 
+from prometheus_client import start_http_server, CollectorRegistry
+
 from powerapi.database.driver import WritableDatabase
 from powerapi.database.exceptions import ConnectionFailed, WriteFailed
 from powerapi.database.prometheus.codecs import ReportEncoders, EncoderOptions
 from powerapi.database.prometheus.collectors import ReportProcessorFactory
 from powerapi.report import Report
 
-try:
-    from prometheus_client import start_http_server, CollectorRegistry
-except ImportError:
-    start_http_server = None
-    CollectorRegistry = None
-
 
 class Prometheus(WritableDatabase):
     """