Your example code has a lot of syntax errors. If I'm interpreting it correctly, then something like this should work.
import arcpy
import datetime
fields = ['date_diag','periodicity','maj_previsional']
expression = "date_diag <> '' and date_diag is not null"
with arcpy.da.UpdateCursor(inFeatures, fields, expression) as cursor:
for date_diag, periodicity, maj_previsional in cursor:
if date_diag:
maj_previsional = maj_previsional + datetime.timedelta(days = periodicity * 365)
cursor.updateRow([date_diag, periodicity, maj_previsional])
If you need to account for leap year/day, then you'll have to update the date calculation.