Turn on mpyp checks for filesystem

[abergmeier]

• Needs some explicit asserts
• Use BinaryIO.closed as property

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Choose reviewer(s) and mention them in a comment (R: @username).
• Format the pull request title like [BEAM-XXX] Fixes bug in ApproximateQuantiles, where you replace BEAM-XXX with the appropriate JIRA issue, if applicable. This will automatically link the pull request to the issue.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

Post-Commit Tests Status (on master branch)

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

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.

[abergmeier]

R: @pabloem

[pabloem]

Run Python PreCommit

[pabloem]

LGTM! Thanks @abergmeier

[codecov]

Codecov Report

Merging #14425 (9d22d4b) into master (bee495f) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #14425      +/-   ##
==========================================
+ Coverage   83.45%   83.48%   +0.02%     
==========================================
  Files         463      447      -16     
  Lines       59013    58883     -130     
==========================================
- Hits        49252    49158      -94     
+ Misses       9761     9725      -36     

Impacted Files	Coverage Δ
...beam/portability/api/beam_artifact_api_pb2_grpc.py
...am/examples/complete/juliaset/juliaset/__init__.py
.../srcs/sdks/python/apache_beam/coders/observable.py
...s/python/apache_beam/examples/wordcount_minimal.py
...beam/testing/load_tests/load_test_metrics_utils.py
.../srcs/sdks/python/apache_beam/utils/proto_utils.py
...cs/sdks/python/apache_beam/typehints/decorators.py
.../sdks/python/apache_beam/examples/snippets/util.py
...sdks/python/apache_beam/internal/metrics/metric.py
...d/srcs/sdks/python/apache_beam/transforms/stats.py
... and 900 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bee495f...9d22d4b. Read the comment docs.