Python requirements are missing wheel

[Mateusz Kowalski]

When pip-installing directly from the source code on a clean python venv, the following appears

```
mko@ubuntu2004:~/git/diskimage-builder$ python3 -m venv testing-venv
mko@ubuntu2004:~/git/diskimage-builder$ source testing-venv/bin/activate
(testing-venv) mko@ubuntu2004:~/git/diskimage-builder$ pip install -r requirements.txt
Collecting networkx>=1.10
  Using cached networkx-2.5-py3-none-any.whl (1.6 MB)
Collecting pbr!=2.1.0,>=2.0.0
  Using cached pbr-5.5.1-py2.py3-none-any.whl (106 kB)
Collecting PyYAML>=3.12
  Using cached PyYAML-5.4.1-cp38-cp38-manylinux1_x86_64.whl (662 kB)
Collecting stevedore>=1.20.0
  Using cached stevedore-3.3.0-py3-none-any.whl (49 kB)
Collecting flake8<4.0.0,>=3.6.0
  Using cached flake8-3.8.4-py2.py3-none-any.whl (72 kB)
Collecting decorator>=4.3.0
  Using cached decorator-4.4.2-py2.py3-none-any.whl (9.2 kB)
Collecting pyflakes<2.3.0,>=2.2.0
  Using cached pyflakes-2.2.0-py2.py3-none-any.whl (66 kB)
Collecting mccabe<0.7.0,>=0.6.0
  Using cached mccabe-0.6.1-py2.py3-none-any.whl (8.6 kB)
Collecting pycodestyle<2.7.0,>=2.6.0a1
  Using cached pycodestyle-2.6.0-py2.py3-none-any.whl (41 kB)
Installing collected packages: decorator, networkx, pbr, PyYAML, stevedore, pyflakes, mccabe, pycodestyle, flake8
Successfully installed PyYAML-5.4.1 decorator-4.4.2 flake8-3.8.4 mccabe-0.6.1 networkx-2.5 pbr-5.5.1 pycodestyle-2.6.0 pyflakes-2.2.0 stevedore-3.3.0
(testing-venv) mko@ubuntu2004:~/git/diskimage-builder$ pip install .
Processing /home/mko/git/diskimage-builder
Requirement already satisfied: PyYAML>=3.12 in ./testing-venv/lib/python3.8/site-packages (from diskimage-builder==3.7.1.dev2) (5.4.1)
Requirement already satisfied: flake8<4.0.0,>=3.6.0 in ./testing-venv/lib/python3.8/site-packages (from diskimage-builder==3.7.1.dev2) (3.8.4) Requirement already satisfied: networkx>=1.10 in ./testing-venv/lib/python3.8/site-packages (from diskimage-builder==3.7.1.dev2) (2.5)
Requirement already satisfied: pbr!=2.1.0,>=2.0.0 in ./testing-venv/lib/python3.8/site-packages (from diskimage-builder==3.7.1.dev2) (5.5.1)
Requirement already satisfied: stevedore>=1.20.0 in ./testing-venv/lib/python3.8/site-packages (from diskimage-builder==3.7.1.dev2) (3.3.0)
Requirement already satisfied: mccabe<0.7.0,>=0.6.0 in ./testing-venv/lib/python3.8/site-packages (from flake8<4.0.0,>=3.6.0->diskimage-builder==3.7.1.dev2) (0.6.1)
Requirement already satisfied: pyflakes<2.3.0,>=2.2.0 in ./testing-venv/lib/python3.8/site-packages (from flake8<4.0.0,>=3.6.0->diskimage-builder==3.7.1.dev2) (2.2.0)
Requirement already satisfied: pycodestyle<2.7.0,>=2.6.0a1 in ./testing-venv/lib/python3.8/site-packages (from flake8<4.0.0,>=3.6.0->diskimage-builder==3.7.1.dev2) (2.6.0)
Requirement already satisfied: decorator>=4.3.0 in ./testing-venv/lib/python3.8/site-packages (from networkx>=1.10->diskimage-builder==3.7.1.dev2) (4.4.2)
Building wheels for collected packages: diskimage-builder
  Building wheel for diskimage-builder (setup.py) ... error
  ERROR: Command errored out with exit status 1:
   command: /home/mko/git/diskimage-builder/testing-venv/bin/python3 -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-req-build-bz3kk2qw/setup.py'"'"'; __file__='"'"'/tmp/pip-req-build-bz3kk2qw/setup.py'"'"';f=
getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' bdist_wheel -d /tmp/pip-wheel-nttv0bej
       cwd: /tmp/pip-req-build-bz3kk2qw/
  Complete output (8 lines):
  /home/mko/git/diskimage-builder/testing-venv/lib/python3.8/site-packages/pbr/core.py:131: UserWarning: Unknown distribution option: 'requires_python'
    warnings.warn(msg)
  usage: setup.py [global_opts] cmd1 [cmd1_opts] [cmd2 [cmd2_opts] ...]
     or: setup.py --help [cmd1 cmd2 ...]
     or: setup.py --help-commands
     or: setup.py cmd --help

  error: invalid command 'bdist_wheel'
  ----------------------------------------
  ERROR: Failed building wheel for diskimage-builder
  Running setup.py clean for diskimage-builder Failed to build diskimage-builder
Installing collected packages: diskimage-builder
  Attempting uninstall: diskimage-builder
    Found existing installation: diskimage-builder 3.7.1.dev2
    Uninstalling diskimage-builder-3.7.1.dev2:
      Successfully uninstalled diskimage-builder-3.7.1.dev2
    Running setup.py install for diskimage-builder ... done
Successfully installed diskimage-builder-3.7.1.dev2
(testing-venv) mko@ubuntu2004:~/git/diskimage-builder$
```

It does not seem like a blocking error, but installing "wheel" via pip before installing diskimage-builder makes this error not to fire.

[Mateusz Kowalski (makowals)]

Another way is adding to setup.py wheel so that setup_requires becomes ['pbr>=2.0.0', 'wheel']. Both ways seem to get rid of the error, not sure if any is desired though