allow @ in identifiers

Identifiers can contain the @ symbol, a typical example is
```
put @resources << /ColorSpace @pgfcolorspaces >>
```
as found in documents created with LaTeX package pgf and colorspaces.

At the moment a pdf containing the above will trigger an error:
```
pdfminer.psparser.PSSyntaxError: Invalid dictionary construct: [/'ColorSpace', /b'@', /b'pgfcolorspaces']
```
since @ is parsed as separate token.

pdfminer/psparser.py

@@ -317,7 +317,7 @@ def _parse_main(self, s: bytes, i: int) -> int:
             self._curtoken = c
             self._parse1 = self._parse_float
             return j + 1
-        elif c.isalpha():
+        elif c.isalpha() or c == b"@":
             self._curtoken = c
             self._parse1 = self._parse_keyword
             return j + 1