(PUP-10015) Add support for arbitrary equality to pip provider

[butlern]

Without this patch the pip provider doesn't correctly parse pip freeze
output specified with arbitrary equality.

https://www.python.org/dev/peps/pep-0440/#arbitrary-equality

This is manifest with triple === separating the package name from the
package version, e.g.

$ pip freeze | grep pytz
pytz===2012f

By not correctly parsing this output, puppet tries to continually
install this package on each run.

The patch updates the regex to handle arbitrary equality.

[puppetlabs-jenkins]

Can one of the admins verify this patch?

[puppetcla]

CLA signed by all contributors.

[ciprianbadescu]

LGTM!

[joshcooper]

Verified pytz is only installed once with this change. Previously puppet would install it every time it ran:

root@lpnl99pdpf4ft3o:~/puppet# bundle exec puppet apply -e "package { 'pytz': ensure => '2012f', provider => pip }"
Notice: Compiled catalog for lpnl99pdpf4ft3o.delivery.puppetlabs.net in environment production in 0.32 seconds
Notice: /Stage[main]/Main/Package[pytz]/ensure: created
Notice: Applied catalog in 3.77 seconds
root@lpnl99pdpf4ft3o:~/puppet# bundle exec puppet apply -e "package { 'pytz': ensure => '2012f', provider => pip }"
Notice: Compiled catalog for lpnl99pdpf4ft3o.delivery.puppetlabs.net in environment production in 0.32 seconds
Notice: Applied catalog in 1.41 seconds
root@lpnl99pdpf4ft3o:~/puppet# bundle exec puppet resource package pytz --param provider
package { 'pytz':
  ensure   => '2012f',
  provider => 'pip',
}

[joshcooper]

Waiting to merge until 6.9.0 tag is created.

[joshcooper]

Thank you for your contribution @butlern! I retargeted this for 6.4.x, and opened a new PR #7713.