PERF: Memory leaks after migrating from 0.25.3 to 1.1.3
hi,
#### Code Sample,

```     
        step = 100
        requestlist = []
        headers = {'Content-type': 'application/json', 'Accept': 'text/plain'}

        for i in range(0, len(df), step):

            tmp_df = df.iloc[i:i + step].copy()

            final_json_string = []

            tmp_df.apply(lambda row: row_to_json_str(row), axis=1)

            final_json_string = "\n".join(final_json_string)
            final_json_string += "\n"

            url = f'{ES_URL}/{table_name}/_bulk'
            requestlist.append(
                grequests.post(url, headers=headersdata=final_json_string))
            del tmp_df
            gc.collect()

        r = grequests.imap(requestlist, size=4, exception_handler=exception_handler)

del final_json_string
del requestlist
del df
cnx.close()
gc.collect()
```

#### Problem description

os : Ubuntu 16.04.7 LTS
Python 3.8.1 (default, Jan  6 2020, 09:57:21)
[GCC 5.4.0 20160609] on linux

after migrating from 0.25.3 to 1.1.3 the memory usage increase  after each method call

back to 0.25.3 the script the memory usage is stable

the source of dataframe is Pandas.read_sql.
the goal of the script is to convert a dataframe to Elasticsearch json request

the loop below isn't exactly the same but the memory profiler give same result :   1669.2 MiB to 6218.0 MiB when for loop comes

memory_usage 

 108   1669.2 MiB      0.0 MiB           list_df = [df[i:i + n] for i in range(0, df.shape[0], n)]
   109
   110   1669.2 MiB      0.0 MiB           requestlist = []
   111
   112   1669.2 MiB      0.0 MiB           headers = {'Content-type': 'application/json', 'Accept': 'text/plain'}
   113
   116   6218.0 MiB      0.0 MiB           for tmp_df in list_df:
   117   6214.4 MiB      0.0 MiB               final_json_string = []
   118
   119   6215.6 MiB      1.5 MiB               tmp_df.apply(lambda row: row_to_json_str(row), axis=1)
   120
   121   6216.9 MiB      1.3 MiB               final_json_string = "\n".join(final_json_string)
   122   6218.0 MiB      1.3 MiB               final_json_string += "\n"
   123
   124   6218.0 MiB      0.2 MiB               url = f'{ES_URL}/{table_name}/_bulk'
   125
   127
   128   6218.0 MiB      0.0 MiB               requestlist.append(
   129   6218.0 MiB      0.2 MiB                   grequests.post(url, headers=headers, data=final_json_string))
   130
   132   6218.0 MiB      0.0 MiB           grequests.imap(requestlist, size=4, exception_handler=exception_handler)
   133   6218.0 MiB      0.0 MiB           del tmp_df
   134   6218.0 MiB      0.0 MiB           del requestlist
   135   6218.0 MiB      0.0 MiB           del list_df

   137   6218.0 MiB      0.0 MiB           del df

   139   6218.0 MiB      0.0 MiB           cnx.close()

thanks 