RFE: remove use `PY2` and `PY3` in the module code

[kloczek]

Looks like some code can be removed now

[tkloczko@pers-jacek wrapt-1.16.0]$ grep -rE "PY2|PY3"
src/wrapt/decorators.py:PY2 = sys.version_info[0] == 2
src/wrapt/decorators.py:if PY2:
src/wrapt/decorators.py:    if PY2:
src/wrapt/decorators.py:    if PY2:
src/wrapt/decorators.py:    if PY2:
src/wrapt/importer.py:PY2 = sys.version_info[0] == 2
src/wrapt/importer.py:if PY2:
src/wrapt/patches.py:PY2 = sys.version_info[0] == 2
src/wrapt/patches.py:if PY2:
src/wrapt/wrappers.py:PY2 = sys.version_info[0] == 2
src/wrapt/wrappers.py:if PY2:
src/wrapt/wrappers.py:    if not PY2:
src/wrapt/wrappers.py:    if not PY2:
tests/compat.py:PY2 = sys.version_info[0] < 3
tests/compat.py:PY3 = sys.version_info[0] >= 3
tests/compat.py:if PY3:
tests/test_adapter.py:from compat import PY2, exec_, getfullargspec
tests/test_adapter.py:        if PY2:
tests/test_adapter_py3.py:from compat import PY2, exec_
tests/test_adapter_py3.py:        if PY2:
tests/test_adapter_py33.py:from compat import PY2, PY3, exec_
tests/test_adapter_py33.py:        if PY2:
tests/test_class.py:from compat import PY2, PY3, exec_
tests/test_class_py37.py:from compat import PY2, PY3, exec_
tests/test_function_wrapper.py:from compat import PY2, PY3, exec_
tests/test_object_proxy.py:from compat import PY2, PY3, exec_
tests/test_object_proxy.py:        if PY3:
tests/test_object_proxy.py:        if PY3:
tests/test_object_proxy.py:        if PY3:
tests/test_object_proxy.py:        if not PY3:
tests/test_object_proxy.py:        if PY3:
tests/test_post_import_hooks.py:from compat import PY2, PY3
tests/test_post_import_hooks.py:        if PY2:
tests/test_update_attributes.py:from compat import PY2, PY3, exec_
tests/test_update_attributes.py:        if PY3:
tests/test_update_attributes.py:        if PY3:
tests/test_update_attributes.py:        if PY3:
tests/test_update_attributes.py:        if PY3: