Add parse_expr to the main __init__.py

[asmeurer]

It's basically been public API for a while, since it's the only way to access
the advanced parsing features like the input transformers.

I've not added the transformers themselves. They can still be accessed from
the sympy_parser submodule. It seems to me that adding them to init.py as
well would pollute the namespace, although I don't feel strongly about it.

References to other Issues or PRs

Brief description of what is fixed or changed

Other comments

Release Notes

• parsing
  • parse_expr can now be imported directly from sympy (from sympy import parse_expr). The various parsing transformers should still be imported from sympy.parsing.sympy_parser.

[sympy-bot]

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• parsing
  • parse_expr can now be imported directly from sympy (from sympy import parse_expr). The various parsing transformers should still be imported from sympy.parsing.sympy_parser. (#18074 by @asmeurer)

This will be added to https://github.com/sympy/sympy/wiki/Release-Notes-for-1.6.

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It's basically been public API for a while, since it's the only way to access
the advanced parsing features like the input transformers.

I've not added the transformers themselves. They can still be accessed from
the sympy_parser submodule. It seems to me that adding them to __init__.py as
well would pollute the namespace, although I don't feel strongly about it.

<!-- Your title above should be a short description of what
was changed. Do not include the issue number in the title. -->

#### References to other Issues or PRs
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#### Brief description of what is fixed or changed


#### Other comments


#### Release Notes

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<!-- BEGIN RELEASE NOTES -->
- parsing
  - `parse_expr` can now be imported directly from sympy (`from sympy import parse_expr`). The various parsing transformers should still be imported from `sympy.parsing.sympy_parser`. 
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[codecov]

Codecov Report

Merging #18074 into master will increase coverage by 0.017%.
The diff coverage is 100%.

@@              Coverage Diff              @@
##            master    #18074       +/-   ##
=============================================
+ Coverage   74.863%   74.881%   +0.017%     
=============================================
  Files          640       641        +1     
  Lines       166561    166564        +3     
  Branches     39179     39179               
=============================================
+ Hits        124694    124725       +31     
+ Misses       36347     36315       -32     
- Partials      5520      5524        +4