Fix double printing in the string and latex printers

[eric-wieser]

References to other Issues or PRs

This is some preliminary cleanup to prepare for gh-19611

Brief description of what is fixed or changed

LatexPrinter now avoids calling self._print(self._print(x)) in all places reached by the test.
This means that subclasses are free to define _print_str to put quotes around the string, without breaking printing of builtin objects.
It also means printing will be marginally faster.

Other comments

Release Notes

• printing
  • LatexPrinter._print is no longer called on strings which have already been converted to Latex.

[sympy-bot]

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Here is what the release notes will look like:

• printing
  • LatexPrinter._print is no longer called on strings which have already been converted to Latex. (#19614 by @eric-wieser)

This will be added to https://github.com/sympy/sympy/wiki/Release-Notes-for-1.7.

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<!-- Your title above should be a short description of what
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#### References to other Issues or PRs
<!-- If this pull request fixes an issue, write "Fixes #NNNN" in that exact
format, e.g. "Fixes #1234" (see
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This is some preliminary cleanup to prepare for gh-19611

#### Brief description of what is fixed or changed

`LatexPrinter` now avoids calling `self._print(self._print(x))` in all places reached by the test.
This means that subclasses are free to define `_print_str` to put quotes around the string, without breaking printing of builtin objects.
It also means printing will be marginally faster.

#### Other comments


#### Release Notes

<!-- Write the release notes for this release below. See
https://github.com/sympy/sympy/wiki/Writing-Release-Notes for more information
on how to write release notes. The bot will check your release notes
automatically to see if they are formatted correctly. -->

<!-- BEGIN RELEASE NOTES -->
* printing
  * `LatexPrinter._print` is no longer called on strings which have already been converted to Latex.
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[codecov]

Codecov Report

Merging #19614 into master will increase coverage by 11.267%.
The diff coverage is 93.181%.

@@              Coverage Diff               @@
##            master    #19614        +/-   ##
==============================================
+ Coverage   64.420%   75.687%   +11.267%     
==============================================
  Files          656       656                
  Lines       170248    170249         +1     
  Branches     40154     40155         +1     
==============================================
+ Hits        109674    128858     +19184     
+ Misses       54537     35760     -18777     
+ Partials      6037      5631       -406     

[oscarbenjamin]

Looks good