[MRG+1] Fix semi_supervised (#9239)

* Files for my dev environment with Docker

* Fixing label clamping (alpha=0 for hard clamping)

* Deprecating alpha, fixing its value to zero

* Correct way to deprecate alpha for LabelPropagation

The previous way was breaking the test
sklearn.tests.test_common.test_all_estimators

* Detailed info for LabelSpreading's alpha parameter

Based on the original paper.

* Minor changes in the deprecation message

* Improving "deprecated" doc string and raising DeprecationWarning

* Using a local "alpha" in "fit" to deprecate LabelPropagation's alpha

This solution isn't great, but it sets the correct value for alpha
without violating the restrictions imposed by the tests.

* Removal of my development files

* Using sphinx's "deprecated" tag (jnothman's suggestion)

* Deprecation warning: stating that the alpha's value will be ignored

* Use __init__ with alpha=None

* Update what's new

* Try fix RuntimeWarning in test_alpha_deprecation

* DOC Indent deprecation details

* DOC wording

* Update docs

* Change to the one true implementation.

* Add sanity-checked impl. of Label{Propagation,Spreading}

* Raise ValueError if alpha is invalid in LabelSpreading.

* Add a normalizing step before clamping to LabelPropagation.

* Fix flake8 errors.

* Remove duplicate imports.

* DOC Update What's New.

* Specify alpha's value in the error.

* Tidy up tests.

Add a test and add references, where needed.

* Add comment to non-regression test.

* Fix documentation.

* Move check for alpha into fit from __init__.

* Fix corner case of LabelSpreading with alpha=None.

* alpha -> self.variant

* Make Whats_new more explicit.

* Simplify impl. of Label{Propagation,Spreading}.

* variant -> _variant.