Improve install conflict warning

[pradyunsg]

Closes #8546
Resolves #8513

Changes

Installing collected packages: pycodestyle, flake8-import-order, mccabe, pyflakes, flake8
ERROR: flake8 3.5.0 requires mccabe<0.7.0,>=0.6.0, but you'll have mccabe 0.5.3 which is incompatible.
ERROR: flake8 3.5.0 requires pycodestyle<2.4.0,>=2.0.0, but you'll have pycodestyle 2.6.0 which is incompatible.
Successfully installed flake8-3.5.0 flake8-import-order-0.17.1 mccabe-0.5.3 pycodestyle-2.6.0 pyflakes-1.6.0

To

Installing collected packages: pycodestyle, flake8-import-order, pyflakes, mccabe, flake8
ERROR: Pip will install or upgrade your package(s) and its dependencies without taking into account other packages you already have installed. This may cause an uncaught dependency conflict. If you would like pip to take your other packages into account, please tell us here: https://forms.gle/cWKMoDs8sUVE29hz9
flake8 3.5.0 requires mccabe<0.7.0,>=0.6.0, but you'll have mccabe 0.5.3 which is incompatible.
flake8 3.5.0 requires pycodestyle<2.4.0,>=2.0.0, but you'll have pycodestyle 2.6.0 which is incompatible.
Successfully installed flake8-3.5.0 flake8-import-order-0.17.1 mccabe-0.5.3 pycodestyle-2.6.0 pyflakes-1.6.0

[pradyunsg]

I think this doesn't need dedicated tests, since (1) I'm lazy (2) see 1 and (3) this is an error messaging change and I'm genuinely not sure if we need additional tests for this.

Oh, and because I'm bored, here's before/after screenshots, from a 80-char wide terminal:

Before:

After:

Honestly, I don't like how either output looks but that's maybe more #4649 thing. :)

[ei8fdb]

I've posted recommendation on in #8546 what the output message should be, how it should be formatted and how a portion should be hidden after 48 hours.

[pradyunsg]

Okay, looks like past me wrote a lot more tests than I'd thought. :)

I'll let the CI run and come back after lunch.

[pradyunsg]

E AssertionError: regex ['c==1\.0\.0', 'incompatible'] not found in stderr: u'DEPRECATION: Python 2.7 reached the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 is no longer maintained. pip 21.0 will drop support for Python 2.7 in January 2021. More details about Python 2 support in pip can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support\nERROR: Exception:\nTraceback (most recent call last):\n File "/tmp/pytest-of-vsts/pytest-1/popen-gw1/pip0/pip/src/pip/_internal/cli/base_command.py", line 216, in _main\n status = self.run(options, args)\n File "/tmp/pytest-of-vsts/pytest-1/popen-gw1/pip0/pip/src/pip/_internal/cli/req_command.py", line 182, in wrapper\n return func(self, options, args)\n File "/tmp/pytest-of-vsts/pytest-1/popen-gw1/pip0/pip/src/pip/_internal/commands/install.py", line 450, in run\n new_resolver='2020-resolver' in options.features_enabled,\n File "/tmp/pytest-of-vsts/pytest-1/popen-gw1/pip0/pip/src/pip/_internal/commands/install.py", line 561, in _warn_about_conflicts\n elif not today_is_later_than(year=2020, month=7, day=31):\n File "/tmp/pytest-of-vsts/pytest-1/popen-gw1/pip0/pip/src/pip/_internal/utils/datetime.py", line 9, in today_is_later_than\n today = datetime.date.today()\nAttributeError: 'module' object has no attribute 'date'\n'

OH COME ON.

Addressed.

[pradyunsg]

Retriggered the CI -- hopefully it'll not fail again because PyPI wasn't responding to XMLRPC for a minute.

[pradyunsg]

Thanks for the review @pfmoore!