This requirements format does not conform to PEP-508. Currently the
extras specified like this work by accident (because _strip_extras()
also parses them). The version checks end up being done with a
misparsed version '1.0[extra]' -- this is not changed in this commit.
Add deprecation warning and fix the corresponding resolver test. Add a
command line test.
Note that we really only check that the Requirement has SpecifierSet
with a specifier that ends in a ']'. A valid version number cannot
contain ']' and no wheels currently on pypi have versions ending in ']'.
Jussi Kukkonen