[typing] enable Pyre strict mode by default

[jimmylai]

Summary

Enable Pyre strict mode by default.

1. Add pyre-fixme and pyre-unsafe to existing type errors and modules.
2. Remove pyre-strict by codemod python3 -m libcst.tool codemod remove_pyre_directive.RemovePyreStrictCommand libcst
3. Update codegen to not adding pyre-strct in generated files.
4. Run test_pyre_integration.py to generate Pyre test fixtures. (The source code has a pyre-strict removal change.)

Test Plan

CI builds.

[carljm]

Seems CI is not happy.

[codecov-commenter]

Codecov Report

Merging #313 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #313      +/-   ##
==========================================
- Coverage   93.96%   93.96%   -0.01%     
==========================================
  Files         223      223              
  Lines       21441    21438       -3     
==========================================
- Hits        20148    20145       -3     
  Misses       1293     1293              

Impacted Files	Coverage Δ
libcst/__init__.py	100.00% <ø> (ø)
libcst/_add_slots.py	85.71% <ø> (ø)
libcst/_batched_visitor.py	94.54% <ø> (ø)
libcst/_exceptions.py	98.70% <ø> (ø)
libcst/_maybe_sentinel.py	100.00% <ø> (ø)
libcst/_metadata_dependent.py	92.30% <ø> (ø)
libcst/_nodes/__init__.py	100.00% <ø> (ø)
libcst/_nodes/base.py	95.29% <ø> (ø)
libcst/_nodes/deep_equals.py	96.29% <ø> (ø)
libcst/_nodes/expression.py	96.62% <ø> (ø)
... and 202 more

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[carljm]

Lgtm!