BUG: 'tmt tests export' is not compatible with python-3.6

[The-Mule]

Runing tmt tests epxport on RHEL-8 fails with the following traceback:

Traceback (most recent call last):
  File "/usr/bin/tmt", line 8, in <module>
    tmt.cli.main()
  File "/usr/lib/python3.6/site-packages/click/core.py", line 721, in __call__
    return self.main(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/click/core.py", line 696, in main
    rv = self.invoke(ctx)
  File "/usr/lib/python3.6/site-packages/click/core.py", line 1065, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/lib/python3.6/site-packages/click/core.py", line 1065, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/lib/python3.6/site-packages/click/core.py", line 894, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/lib/python3.6/site-packages/click/core.py", line 534, in invoke
    return callback(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/usr/lib/python3.6/site-packages/tmt/cli.py", line 427, in export
    test.export(format_='nitrate', create=create, general=general)
  File "/usr/lib/python3.6/site-packages/tmt/base.py", line 318, in export
    tmt.export.export_to_nitrate(self, create, general)
  File "/usr/lib/python3.6/site-packages/tmt/export.py", line 168, in export_to_nitrate
    fmf_id = test.fmf_id
  File "/usr/lib/python3.6/site-packages/tmt/base.py", line 95, in fmf_id
    origin = run('git config --get remote.origin.url')
  File "/usr/lib/python3.6/site-packages/tmt/base.py", line 89, in run
    result = subprocess.run(command.split(), capture_output=True)
  File "/usr/lib64/python3.6/subprocess.py", line 423, in run
    with Popen(*popenargs, **kwargs) as process:
TypeError: __init__() got an unexpected keyword argument 'capture_output'

It turns out that this issue is related to difference between python 3.6 and 3.7. Please notice that RHEL-8 contains python.3.6 only.