Revert unnecessary runtime check for valid Python interpreter (#7451)

### Problem
Pants developers run Pants differently than end users do. Whereas they either download the wheel from PyPi or use a Pex, developers build from source. This allows us to run Pants with any Python version we'd like.

Due to a misunderstanding thinking this meant end users could also run Pants with any Python interpreter they wanted, we added in #7365 a runtime check to ensure they use 2.7 or 3.6+.

However, we realized it is impossible (or at least extremely unlikely) that end users will run Pants with an invalid interpreter, due to install time checks, as follows:

* Pip will not resolve for invalid interpreters like 3.5, as it will not be able to find a valid wheel.
* Pex includes interpreter constraints when being built, so the Pex cannot be ran with an invalid interpreter.

Thus, the runtime check is unnecessary.

### Solution
Remove the runtime check.

### Result
Less code and slightly faster performance due to one less check.

Author: Eric-Arellano

src/python/pants/bin/pants_loader.py

@@ -7,7 +7,6 @@
 import importlib
 import locale
 import os
-import sys
 import warnings
 from builtins import object
 from textwrap import dedent
@@ -20,19 +19,10 @@ class PantsLoader(object):
   DEFAULT_ENTRYPOINT = 'pants.bin.pants_exe:main'
 
   ENCODING_IGNORE_ENV_VAR = 'PANTS_IGNORE_UNRECOGNIZED_ENCODING'
-  INTERPRETER_IGNORE_ENV_VAR = 'PANTS_IGNORE_UNSUPPORTED_PYTHON_INTERPRETER'
 
   class InvalidLocaleError(Exception):
     """Raised when a valid locale can't be found."""
 
-  class InvalidInterpreter(Exception):
-    """Raised when trying to run Pants with an unsupported Python version."""
-
-  @staticmethod
-  def is_supported_interpreter(major_version, minor_version):
-    return (major_version == 2 and minor_version == 7) \
-      or (major_version == 3 and minor_version >= 6)
-
   @staticmethod
   def setup_warnings():
     # We want to present warnings to the user, set this up before importing any of our own code,
@@ -72,27 +62,6 @@ def ensure_locale(cls):
         """.format(encoding, cls.ENCODING_IGNORE_ENV_VAR)
       ))
 
-  @classmethod
-  def ensure_valid_interpreter(cls):
-    """Runtime check that user is using a supported Python version."""
-    py_major, py_minor = sys.version_info[0:2]
-    if not PantsLoader.is_supported_interpreter(py_major, py_minor) and os.environ.get(cls.INTERPRETER_IGNORE_ENV_VAR, None) is None:
-      raise cls.InvalidInterpreter(dedent("""
-        You are trying to run Pants with Python {}.{}, which is unsupported.
-        Pants requires a Python 2.7 or a Python 3.6+ interpreter to be
-        discoverable on your PATH to run.
-
-        If you still get this error after ensuring at least one of these interpreters
-        is discoverable on your PATH, you may need to modify your build script
-        (e.g. `./pants`) to properly set up a virtual environment with the correct
-        interpreter. We recommend following our setup guide and using our setup script
-        as a starting point: https://www.pantsbuild.org/setup_repo.html.
-
-        Alternatively, you may bypass this error by setting the below environment variable.
-          {}=1
-        Note: we cannot guarantee consistent behavior with this bypass enabled.
-        """.format(py_major, py_minor, cls.INTERPRETER_IGNORE_ENV_VAR)))
-
   @staticmethod
   def determine_entrypoint(env_var, default):
     return os.environ.pop(env_var, default)
@@ -109,7 +78,6 @@ def load_and_execute(entrypoint):
   @classmethod
   def run(cls):
     cls.setup_warnings()
-    cls.ensure_valid_interpreter()
     cls.ensure_locale()
     entrypoint = cls.determine_entrypoint(cls.ENTRYPOINT_ENV_VAR, cls.DEFAULT_ENTRYPOINT)
     cls.load_and_execute(entrypoint)

tests/python/pants_test/bin/BUILD

@@ -1,18 +1,8 @@
 # Copyright 2015 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-python_tests(
-  sources='test_loader.py',
-  dependencies=[
-    'src/python/pants/bin',
-    '3rdparty/python:future',
-    '3rdparty/python:mock',
-  ],
-)
 
 python_tests(
-  name='integration',
-  sources='test_loader_integration.py',
   dependencies=[
     'src/python/pants/bin',
     'tests/python/pants_test:int-test',