[CssSelector] added the license of the Python library we ported to PHP
minor #13104 [CssSelector] added the license of the Python library we ported to PHP (fabpot)

This PR was merged into the 2.3 branch.

Discussion
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[CssSelector] added the license of the Python library we ported to PHP

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11894
| License       | MIT
| Doc PR        | n/a

Commits
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fa36b98 [CssSelector] added the license of the Python library we ported to PHP