Style fixes for GowinPlatform

- div_frac.numerator instead of int(div_frac) for clarity
- `import re` as in other platforms
- use `re.match` instead of `re.search`+`len(match.groups())==n`
- use r"" for regexps for syntax highlighting and to ensure invalid
  escapes do not break on later Python versions

amaranth/vendor/gowin.py

@@ -1,6 +1,6 @@
 from abc import abstractproperty
 from fractions import Fraction
-from re import search
+import re
 
 from ..hdl import *
 from ..lib.cdc import ResetSynchronizer
@@ -47,16 +47,17 @@ class GowinPlatform(TemplatedPlatform):
 
     def parse_part(self):
         # These regular expressions match all >900 parts of Gowin device_info.csv
-        reg_series    = "(GW[12]{1}[AN]{1}[EFNRSZ]{0,3})-"
-        reg_voltage   = "(ZV|EV|LV|LX|UV|UX)"
-        reg_size      = "(1|2|4|9|18|55)"
-        reg_subseries = "(?:(B|C|S|X|P5)?)"
-        reg_package   = "((?:PG|UG|EQ|LQ|MG|M|QN|CS|FN)(?:\d+)(?:P?)(?:A|E|M|CF|C|D|G|H|F|S|T|U|X)?)"
-        reg_speed     = "((?:C\d{1}/I\d{1})|ES|A\d{1}|I\d{1})"
-
-        match = search(reg_series+reg_voltage+reg_size+reg_subseries+reg_package+reg_speed, self.part)
-        if len(match.groups()) != 6:
-            raise ValueError("Supplied part name is invalid.")
+        reg_series    = r"(GW[12]{1}[AN]{1}[EFNRSZ]{0,3})-"
+        reg_voltage   = r"(ZV|EV|LV|LX|UV|UX)"
+        reg_size      = r"(1|2|4|9|18|55)"
+        reg_subseries = r"(?:(B|C|S|X|P5)?)"
+        reg_package   = r"((?:PG|UG|EQ|LQ|MG|M|QN|CS|FN)(?:\d+)(?:P?)(?:A|E|M|CF|C|D|G|H|F|S|T|U|X)?)"
+        reg_speed     = r"((?:C\d{1}/I\d{1})|ES|A\d{1}|I\d{1})"
+
+        match = re.match(reg_series+reg_voltage+reg_size+reg_subseries+reg_package+reg_speed+"$",
+                         self.part)
+        if not match:
+            raise ValueError("Supplied part name is invalid")
 
         self.series    = match.group(1)
         self.voltage   = match.group(2)
@@ -65,9 +66,9 @@ def parse_part(self):
         self.package   = match.group(5)
         self.speed     = match.group(6)
 
-        match = search(reg_series+reg_size+reg_subseries, self.family)
-        if len(match.groups()) != 3:
-            raise ValueError("Supplied device family name is invalid.")
+        match = re.match(reg_series+reg_size+reg_subseries+"$", self.family)
+        if not match:
+            raise ValueError("Supplied device family name is invalid")
 
         self.series_f    = match.group(1)
         self.size_f      = match.group(2)
@@ -76,9 +77,11 @@ def parse_part(self):
         # subseries_f is usually more reliable than subseries.
 
         if self.series != self.series_f:
-            raise ValueError("Series extracted from supplied part name does not match supplied family series")
+            raise ValueError("Series extracted from supplied part name does not match "
+                             "supplied family series")
         if self.size != self.size_f:
-            raise ValueError("Size extracted from supplied part name does not match supplied family size")
+            raise ValueError("Size extracted from supplied part name does not match "
+                             "supplied family size")
 
     # _chipdb_device is tied to available chipdb-*.bin files of nextpnr-gowin
     @property
@@ -172,7 +175,7 @@ def _osc_div(self):
                 "steps of {}"
                 .format(div_frac.numerator, div_frac.denominator, div_min, div_max, div_step))
 
-        return int(div_frac)
+        return div_frac.numerator
 
     # Common templates