Allow skipping DataFrameClient import

Importing the DataFrameClient can take quite a while,
especially when the source files aren't yet loaded into
the page cache.

This patch adds an environment variable, INFLUXDB_NO_DATAFRAME_CLIENT,
to allow users of this package to skip this import in cases where
they don't need it.

Author: doyle-arista

influxdb/__init__.py

@@ -17,7 +17,7 @@
     'SeriesHelper',
 ]
 
-if os.environ.get("INFLUXDB_NO_DATAFRAME_CLIENT", "0").lower() not in ("0", "false"):
+if os.environ.get("INFLUXDB_NO_DATAFRAME_CLIENT", "0").lower() not in ("1", "true"):
    from .dataframe_client import DataFrameClient
    __all__.append( "DataFrameClient" )