remove usage of php alias in the python client

[melange396]

Get rid of the use of the "api.php" generic compatibility alias in the python delphi_epidata client, referenced by the BASE_URL class variable. Do this by changing the client to use the proper URL path suffix for each of the individual endpoints instead of specifying them in the endpoint parameter on calls to _request().

This has a number of benefits, including:

• a higher upper limit on the number of rows returned per request (see limits here)
• better/easier logging aggregation and analysis (some request parameter values are used differently in the generic compatibility (php alias) endpoint vs in the other endpoints when used directly)
• movement toward finally deprecating and removing the php alias endpoint entirely

The client is used by a number of integration tests in this repository. Externally, the client is used in the covidcast-indicators repo, and built upon in the core of the covidcast client. Changes may need to be made to those as well to ensure they support any edits made to the client.