Pandas 0.25.x drops support for Python 2.7

[aerispaha]

This is causing an issue in Python 2.7 Windows builds. We should either drop support for Python 2.7 or set the Pandas version explicitly to v0.24.2. This is causing a build fail in #63.

Processing pandas-0.25.0.tar.gz
Writing c:\users\appveyor\appdata\local\temp\1\easy_install-4dl70z\pandas-0.25.0\setup.cfg
Running pandas-0.25.0\setup.py -q bdist_egg --dist-dir c:\users\appveyor\appdata\local\temp\1\easy_install-4dl70z\pandas-0.25.0\egg-dist-tmp-7pcggv
Traceback (most recent call last):
  File "setup.py", line 68, in <module>
    "Programming Language :: Python :: 3.6",
  File "C:\Python27\lib\site-packages\setuptools\__init__.py", line 143, in setup
    return distutils.core.setup(**attrs)
  File "C:\Python27\lib\distutils\core.py", line 151, in setup
    dist.run_commands()
  File "C:\Python27\lib\distutils\dist.py", line 953, in run_commands
    self.run_command(cmd)
  File "C:\Python27\lib\distutils\dist.py", line 972, in run_command
    cmd_obj.run()
  File "C:\Python27\lib\site-packages\setuptools\command\develop.py", line 38, in run
    self.install_for_development()
  File "C:\Python27\lib\site-packages\setuptools\command\develop.py", line 154, in install_for_development
    self.process_distribution(None, self.dist, not self.no_deps)
  File "C:\Python27\lib\site-packages\setuptools\command\easy_install.py", line 752, in process_distribution
    [requirement], self.local_index, self.easy_install
  File "C:\Python27\lib\site-packages\pkg_resources\__init__.py", line 780, in resolve
    replace_conflicting=replace_conflicting
  File "C:\Python27\lib\site-packages\pkg_resources\__init__.py", line 1063, in best_match
    return self.obtain(req, installer)
  File "C:\Python27\lib\site-packages\pkg_resources\__init__.py", line 1075, in obtain
    return installer(requirement)
  File "C:\Python27\lib\site-packages\setuptools\command\easy_install.py", line 679, in easy_install
    return self.install_item(spec, dist.location, tmpdir, deps)
  File "C:\Python27\lib\site-packages\setuptools\command\easy_install.py", line 705, in install_item
    dists = self.install_eggs(spec, download, tmpdir)
  File "C:\Python27\lib\site-packages\setuptools\command\easy_install.py", line 890, in install_eggs
    return self.build_and_install(setup_script, setup_base)
  File "C:\Python27\lib\site-packages\setuptools\command\easy_install.py", line 1158, in build_and_install
    self.run_setup(setup_script, setup_base, args)
  File "C:\Python27\lib\site-packages\setuptools\command\easy_install.py", line 1144, in run_setup
    run_setup(setup_script, args)
  File "C:\Python27\lib\site-packages\setuptools\sandbox.py", line 253, in run_setup
    raise
  File "C:\Python27\lib\contextlib.py", line 35, in __exit__
    self.gen.throw(type, value, traceback)
  File "C:\Python27\lib\site-packages\setuptools\sandbox.py", line 195, in setup_context
    yield
  File "C:\Python27\lib\contextlib.py", line 35, in __exit__
    self.gen.throw(type, value, traceback)
  File "C:\Python27\lib\site-packages\setuptools\sandbox.py", line 166, in save_modules
    saved_exc.resume()
  File "C:\Python27\lib\site-packages\setuptools\sandbox.py", line 141, in resume
    six.reraise(type, exc, self._tb)
  File "C:\Python27\lib\site-packages\setuptools\sandbox.py", line 154, in save_modules
    yield saved
  File "C:\Python27\lib\site-packages\setuptools\sandbox.py", line 195, in setup_context
    yield
  File "C:\Python27\lib\site-packages\setuptools\sandbox.py", line 250, in run_setup
    _execfile(setup_script, ns)
  File "C:\Python27\lib\site-packages\setuptools\sandbox.py", line 45, in _execfile
    exec(code, globals, locals)
  File "c:\users\appveyor\appdata\local\temp\1\easy_install-4dl70z\pandas-0.25.0\setup.py", line 21, in <module>
    return version
  File "c:\users\appveyor\appdata\local\temp\1\easy_install-4dl70z\pandas-0.25.0\versioneer.py", line 1629
    print("Adding sample versioneer config to setup.cfg", file=sys.stderr)
                                                              ^
SyntaxError: invalid syntax
Command exited with code 1