[BEAM-12399] Move CPython license to own file.

[lostluck]

In order to resolve BEAM-12399 (Beam Go SDK docs can't be displayed on the standard GoDoc site pkg.go.dev) we need to separate out the CPython license stack from the other licenses which apply more broadly.

• Moves that license text from LICENSE to LICENSE.python
• Ensures the LICENSE.python file is also copied to the Python SDK distribution images.

Per golang/go#45095, and #46291 the issue is that the license not particularly compatible, so adding it as a recognized license is apparently not the way forward. However, as the license only needs to be posted with distributions of cpython, it largely only needs to go into the Python SDK container(s).

Since pkg.go.dev must be conservative about what it checks for LICENSE we can't pare down or filter what the site picks up (eg. a LICENSE.golang or a LICENSE in the sdks/go subdirectory file would only be added to whats in the root LICENSE, not supercede it). However, pkg.go.dev doesn't look at other LICENSE files for particular languages, hence the new file.

The way we're referring to the LICENSE.python sidecar file is also acceptable to Apache Legal https://issues.apache.org/jira/browse/LEGAL-582

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ValidatesRunner compliance status (on master branch)

Lang	ULR	Dataflow	Flink	Samza	Spark	Twister2
Go	---					---
Java
Python	---					---
XLang						---

Examples testing status on various runners

Lang	ULR	Dataflow	Flink	Samza	Spark	Twister2
Go	---	---	---	---	---	---	---
Java	---		---	---	---	---	---
Python	---	---	---	---	---	---	---
XLang	---	---	---	---	---	---	---

Post-Commit SDK/Transform Integration Tests Status (on master branch)

Go	Java	Python

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website	Whitespace	Typescript
Non-portable
Portable	---			---	---	---

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

GitHub Actions Tests Status (on master branch)

See CI.md for more information about GitHub Actions CI.

[codecov]

Codecov Report

Merging #15201 (618a2ab) into master (46842eb) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #15201      +/-   ##
==========================================
- Coverage   83.82%   83.81%   -0.02%     
==========================================
  Files         441      441              
  Lines       59706    59706              
==========================================
- Hits        50051    50044       -7     
- Misses       9655     9662       +7     

Impacted Files	Coverage Δ
.../python/apache_beam/testing/test_stream_service.py	88.37% <0.00%> (-4.66%)	⬇️
sdks/python/apache_beam/utils/interactive_utils.py	87.80% <0.00%> (-2.44%)	⬇️
...hon/apache_beam/runners/direct/test_stream_impl.py	91.79% <0.00%> (-2.24%)	⬇️
...che_beam/runners/interactive/interactive_runner.py	90.65% <0.00%> (-1.87%)	⬇️
.../python/apache_beam/transforms/periodicsequence.py	96.72% <0.00%> (-1.64%)	⬇️
...hon/apache_beam/runners/worker/bundle_processor.py	93.51% <0.00%> (-0.13%)	⬇️
sdks/python/apache_beam/io/source_test_utils.py	89.86% <0.00%> (+1.38%)	⬆️

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