Make /usr/bin/pip(3) work with user-installed pip 19.3+

Resolves https://bugzilla.redhat.com/show_bug.cgi?id=1767212

== Before:

$ rpm -q python3-pip
python3-pip-19.2.3-1.fc32.noarch

$ /usr/bin/pip --version
pip 19.2.3 from /usr/lib/python3.8/site-packages/pip (python 3.8)

$ pip install --user --upgrade pip
...
Successfully installed pip-19.3.1

$ /usr/bin/pip --version
Traceback (most recent call last):
  File "/usr/bin/pip", line 15, in <module>
    sys.exit(main())
TypeError: 'module' object is not callable

$ rm .local/ -rf
$ pip install --user --upgrade 'pip<10'
...
Successfully installed pip-9.0.3

$ /usr/bin/pip --version
pip 9.0.3 from /home/pythonista/.local/lib/python3.8/site-packages (python 3.8)

$ pip install --user --upgrade 'pip<9'
...
Successfully installed pip-8.1.2

$ /usr/bin/pip --version
pip 8.1.2 from /home/pythonista/.local/lib/python3.8/site-packages (python 3.8)

== After

$ rpm -q python3-pip
python3-pip-19.2.3-2.fc32.noarch

$ /usr/bin/pip --version
pip 19.2.3 from /usr/lib/python3.8/site-packages/pip (python 3.8)

$ pip install --user --upgrade pip
...
Successfully installed pip-19.3.1

$ /usr/bin/pip --version
pip 19.3.1 from /home/pythonista/.local/lib/python3.8/site-packages/pip (python 3.8)

$ pip install --user --upgrade 'pip<10'
...
Successfully installed pip-9.0.3

$ /usr/bin/pip --version
pip 9.0.3 from /home/pythonista/.local/lib/python3.8/site-packages (python 3.8)

$ pip install --user --upgrade 'pip<9'
...
Successfully installed pip-8.1.2

$ /usr/bin/pip --version
pip 8.1.2 from /home/pythonista/.local/lib/python3.8/site-packages (python 3.8)

pip-allow-different-versions.patch

@@ -0,0 +1,20 @@
+--- /usr/bin/pip3	2018-03-29 15:22:13.000000000 +0200
++++ pip3	2018-05-04 11:49:08.098821010 +0200
+@@ -4,7 +4,16 @@
+ import re
+ import sys
+ 
+-from pip._internal import main
++try:
++    from pip._internal import main
++except ImportError:
++    # user has most probably downgraded pip in their home
++    # so let them run it anyway until ~/.local/bin makes it in front of the PATH
++    from pip import main
++else:
++    # user might also upgraded pip...
++    if hasattr(main, 'main'):
++        main = main.main
+ 
+ if __name__ == '__main__':
+     sys.argv[0] = re.sub(r'(-script\.pyw?|\.exe)?$', '', sys.argv[0])