Inserting setuptools and wheel imports at top of setup.py files breaks modules with __future__ imports in setup.py

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Many modules use from __future__ imports in setup.py. An example of this occurring can be seen here:

❯ python setup.py bdist_xar --download
...
Successfully downloaded hvac pyhcl requests certifi charset-normalizer idna urllib3
  File "/tmp/tmpotmbe2tz/pyhcl-0.4.4/setup.py", line 5
    from __future__ import print_function
    ^
SyntaxError: from __future__ imports must occur at the beginning of the file
...

xar/xar/pip_installer.py

Lines 116 to 117 in a67d37c

	f.write("import setuptools\n")
	f.write("import wheel\n")