TST/REF: splitting pandas/io/parsers.py into multiple files

[arw2019]

pandas/io/parsers.py is close to 4k LOC (3986 on 1.3 master). Would it be reasonable to split it into multiple files (located, say, in a pandas/io/parsers directory)?

AFAICT the file contains 4 logical pieces:

• read_... methods (~700 LOC)
• ParserBase and related module-level methods ( ~1100 LOC)
• CParser and related module-level methods (~400LOC)
• PythonParser, its derived classes and related module-level methods (~1800 LOC)

Once #38370 goes in these would be joined by

• PyarrowParser (~100 LOC right now)

IMO this kind of refactor would make the module somewhat easier to grok

[jreback]

sure seems reasonable