Add a new CLI command to lint PO files

[tomasr8]

Partially addresses #1011

This adds a new lint command to babel:

$ pybabel lint --help                                
Usage: pybabel lint [options] <input-paths>

check message catalogs for common problems

Options:
  -h, --help  show this help message and exit

Example output:

$ pybabel lint messages.po
messages.po:20: placeholders in msgid differ from placeholders in msgstr:
	placeholders in msgid but missing in msgstr: {foo}
	placeholders in msgstr but missing in msgid: {bar}

The implementation is based on my team's implementation in Indico.

Some notes:

• Currently only f-string placeholders are checked. I didn't implement the old %-style formatting to keep the diff smaller, but I can add it later.
• There are currently no CLI options besides the input-paths. We might want to consider adding a parameter to ignore a specific file/msgid but that can again be added later.

Feedback welcome!

(@akx would you mind taking a look?)

[codecov]

Codecov Report

Attention: Patch coverage is 93.24324% with 5 lines in your changes missing coverage. Please review.

Project coverage is 91.34%. Comparing base (9d25e67) to head (3248f82).
Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
babel/messages/frontend.py	93.24%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1168      +/-   ##
==========================================
- Coverage   91.47%   91.34%   -0.14%     
==========================================
  Files          27       27              
  Lines        4636     4728      +92     
==========================================
+ Hits         4241     4319      +78     
- Misses        395      409      +14     

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[jun66j5]

Why the new checker is not added to babel/messages/checkers.py? It could check using Message.check() method if added. Also, checker for %-styling formatting is already implemented.

(Side note: Trac provides check_catalog command via setup.py to check catalogs using the method.)

[tomasr8]

Why the new checker is not added to babel/messages/checkers.py?

Yeah, that might be a good idea. I'll look into that :)

[tomasr8]

Perhaps it might make more sense to start with implementing the python-brace-format flag