Introduces tests for 'bdemeta.parse_config'.
Introduces test for 'bdemeta.InvalidArgumentsError'.
Always opens files in default platform encoding.

Prior to this commit, the type of data returned when reading 'bde' metadata
files depended on the major version of 'Python' being used - version 2 returned
byte sequences while version 3 returned code point sequences.  This commit
consistently uses code point sequences across 'Python' versions.