Repository: biopython/biopython

Issue:
Multiple valued comments in TRANSFAC motifs files
### Setup

I am reporting a problem with Biopython version, Python version, and operating
system as follows:

```python
>>> import sys; print(sys.version)
3.11.4 (main, Jun 13 2023, 13:54:09) [GCC 8.3.0]
>>> import platform; print(platform.python_implementation()); print(platform.platform())
CPython
Linux-4.19.0-17-amd64-x86_64-with-glibc2.28
>>> import Bio; print(Bio.__version__)
1.82.dev0
```

### Expected behaviour

TRANSFAC format can be used to store multiple comments, an example are JASPAR's TRANSFAC single batch files.

eg.
```
AC  MA0004.1                                                                          
XX                                                                                    
ID  Arnt                                                                              
XX                                                                                    
DE  MA0004.1 Arnt ; From JASPAR                                                       
PO  A C G T                                                                           
01  4.0 16.0 0.0 0.0                                                                  
02  19.0 0.0 1.0 0.0                                                                  
03  0.0 20.0 0.0 0.0                                                                  
04  0.0 0.0 20.0 0.0                                                                  
05  0.0 0.0 0.0 20.0                                                                  
06  0.0 0.0 20.0 0.0                                                                  
XX                                                                                    
CC  tax_group:vertebrates                                                             
CC  tf_family:PAS domain factors                                                      
CC  tf_class:Basic helix-loop-helix factors (bHLH)                                    
CC  pubmed_ids:7592839                                                                
CC  uniprot_ids:P53762                                                                
CC  data_type:SELEX                                                                   
XX                                                                                    
//
...
```

currently, when parsing only the last comment is captured. The introduced change propose to treat comments as a multiple value key and store all comments in a list.

### Actual behaviour

For multi-line comments in TRANSFAC motif file only last one is stored.

### Steps to reproduce

[JASPAR2022_CORE_vertebrates_non-redundant_pfms_transfac.txt](https://jaspar.genereg.net/download/data/2022/CORE/JASPAR2022_CORE_vertebrates_non-redundant_pfms_transfac.txt)

```
with open("JASPAR2022_CORE_vertebrates_non-redundant_pfms_transfac.txt") as handle:
    motifdb = motifs.parse(handle, "TRANSFAC")
motifdb[0]['CC']
```



Discussion:

Resolving patch:
diff --git a/Bio/motifs/transfac.py b/Bio/motifs/transfac.py
--- a/Bio/motifs/transfac.py
+++ b/Bio/motifs/transfac.py
@@ -62,7 +62,7 @@ class Motif(motifs.Motif, dict):
     For more information, see the TRANSFAC documentation.
     """
 
-    multiple_value_keys = {"BF", "OV", "HP", "BS", "HC", "DT", "DR"}
+    multiple_value_keys = {"BF", "OV", "HP", "BS", "HC", "DT", "DR", "CC"}
     # These keys can occur multiple times for one motif
 
     reference_keys = {"RX", "RA", "RT", "RL"}


Test patch: