python2 error message

[dtrodrigues]

This shows a more informative error message when trying to use get-pip.py with Python 2. Something similar for Python 3.5 and older may also make sense.

Before:

$ python2 get-pip.py
Traceback (most recent call last):
  File "get-pip.py", line 24226, in <module>
    main()
  File "get-pip.py", line 199, in main
    bootstrap(tmpdir=tmpdir)
  File "get-pip.py", line 82, in bootstrap
    from pip._internal.cli.main import main as pip_entry_point
  File "/var/folders/h4/tkxs1nyx46zd3zclshdprkv00000gn/T/tmpIcTK5C/pip.zip/pip/_internal/cli/main.py", line 60
    sys.stderr.write(f"ERROR: {exc}")
                                   ^
SyntaxError: invalid syntax

After:

$ python2 get-pip.py
Python 2 is not supported with this version of get-pip.py. Please use https://bootstrap.pypa.io/2.7/get-pip.py

See also
pypa/pip#9500 (comment)
pypa/pip#9501
pypa/pip#9503
pypa/pip#9513
pypa/pip#9518
pypa/pip#9520
pypa/pip#9574
pypa/pip#9577
pypa/pip#9581
#82
#89
#92
#94
https://discuss.python.org/t/install-pip-on-ubuntu20-04-for-python2/6880
https://stackoverflow.com/questions/65866417/pip-install-failing-on-python2
https://stackoverflow.com/questions/65896334/python-pip-broken-wiith-sys-stderr-writeferror-exc
https://stackoverflow.com/questions/65932497/get-pip-py-returns-syntaxerror-invalid-syntax

[dtrodrigues]

This looks to be a more narrow implementation of #25. I'm fine with either PR being incorporated, but it would be beneficial if the messaging was clearer than the status quo when somebody tries to use the wrong version of get-pip.py.

[mblufkin]

I recently encountered this error and had to switch to the versioned file (2.7), but this error message would have been helpful.

[cclauss]

This usage of an f-string breaks pyenv builds of Python 2.7, 3.4, and 3.5 until pyenv/pyenv#1793 (or similar) is landed and released.