I was trying to use https://github.com/NiklasRosenstein/pydoc-markdown to generate some doc for https://github.com/Mulugruntz/aiosubprocess

It was failing and for a while I thought I was doing something wrong. But when I did dig deeper, I realized that it was failing because I has a method named "exec".

The reason why I want to use "exec" is to make it obvious whether I'm executing the subprocess in shell mode or not (there's also a "shell" method).

As we can see here https://docs.python.org/3.9/reference/lexical_analysis.html#keywords

"exec" is not reserved.

Moreover, it's pretty counterintuitive that the code parses and runs correctly (cpython 3.9.5) but the lib2to3 parser crashes.

See below a working example:

```python
from lib2to3 import pygram, pytree
from lib2to3.pgen2 import driver
from lib2to3.pgen2.parse import ParseError

grammar = pygram.python_grammar.copy()
driver = driver.Driver(grammar, convert=pytree.convert)

strings = [
    "def fname(): pass",
    "def exec(): pass",
    """
class C:
    def exec(self): pass""",
]

for s in strings:
    try:
        driver.parse_string(s + '\n')
    except ParseError as pe:
        print("It fails:", s)
    else:
        print("It works:", s)

```

```shell
It works: def fname(): pass
It fails: def exec(): pass
It fails: 
class C:
    def exec(self): pass
```

'exec' was a keyword in 2.x, but that should not matter in 3.9.  What OS?  Did you get an actual crash (core dump on *nix), or a python exception and traceback (not a crash)?  If the latter, copy and paste it.

Traceback (most recent call last):
  File "/Users/sgiffard/Library/Application Support/JetBrains/PyCharm2020.3/scratches/scratch_24.py", line 9, in <module>
    driver.parse_string("""class C:\n    def exec(self): pass\n""")
  File "/usr/local/Cellar/python@3.9/3.9.5/Frameworks/Python.framework/Versions/3.9/lib/python3.9/lib2to3/pgen2/driver.py", line 103, in parse_string
    return self.parse_tokens(tokens, debug)
  File "/usr/local/Cellar/python@3.9/3.9.5/Frameworks/Python.framework/Versions/3.9/lib/python3.9/lib2to3/pgen2/driver.py", line 71, in parse_tokens
    if p.addtoken(type, value, (prefix, start)):
  File "/usr/local/Cellar/python@3.9/3.9.5/Frameworks/Python.framework/Versions/3.9/lib/python3.9/lib2to3/pgen2/parse.py", line 162, in addtoken
    raise ParseError("bad input", type, value, context)
lib2to3.pgen2.parse.ParseError: bad input: type=1, value='exec', context=(' ', (2, 8))

Sorry, I forgot to mention:
macOS Mojave 10.14.5 (18F132)

Changing type as crash typically refers to segfault and not an exception.