add python-slugify requirement

requirements.in

 entsoe-py
-pycountry
\ No newline at end of file
+pycountry
+python-slugify
\ No newline at end of file

requirements.txt

@@ -13,8 +13,10 @@ numpy==1.19.4             # via pandas
 pandas==1.1.4             # via entsoe-py
 pycountry==20.7.3         # via -r requirements.in
 python-dateutil==2.8.1    # via pandas
+python-slugify==4.0.1     # via -r requirements.in
 pytz==2020.4              # via entsoe-py, pandas
 requests==2.25.0          # via entsoe-py
 six==1.15.0               # via python-dateutil
 soupsieve==2.0.1          # via beautifulsoup4
+text-unidecode==1.3       # via python-slugify
 urllib3==1.26.2           # via requests