Remove Python 2.7 support

[goodmami]

Python 2.7 is nearing it's end-of-life, so removing PyDelphin support for it by that time would be appropriate. There are some pieces of code that currently depend on PyDelphin's Python 2.7 support (e.g., by having Python 2.7-only scripts that import PyDelphin), though, so this issue is for tracking those dependencies.

• JaEn's utils/select-rule.py

Any others?

This issue is also a good place to track the places in the PyDelphin code where we've had to bend to keep Python 2.7 compatibility (see the following places for comments in the code):

• Numerous from __future__ import ... statements
• delphin.util.stringtypes
• delphin.repp._REPPGroupCall._apply()
• delphin.commands._prepare_output_directory()
• delphin.main.redent()
• delphin.itsdb.unicode
• delphin.itsdb._open_table()
• delphin.exceptions.TdlParsingError
• delphin.interfaces.rest.urljoin
• delphin.mrs.util.etree_tostring

Other things to look for are calls to .encode() or .decode(), opportunities to use yield from instead of a loop over yield, NameErrors, etc.